Update Stylelint to v14, extend SCSS plugins, remove primer-* configs, resolve issues (#821)

This is a catch-all PR that modernizes and updates our Stylelint config, and resolves all open issues. This is a pretty big change - so I want to update all of our related dependencies in lockstep.

In particular, this PR

- [x] updates stylelint to `v14`
- [x] adds in the standard stylelint config for SCSS (`stylelint-config-standard-scss`)
- [x] swaps out `stylelint-config-prettier` for `stylelint-config-prettier-scss`
- [x] ~~properly update `@primer`-related plugins:~~ completely remove `primer` from our configuration
- [x] autofix, manually resolve, or disable all newly-introduced lint errors; **I've avoided manually resolving errors that would be a behavioural change**
- [x] re-runs `npm run format`

See the "next steps" section on some extra thoughts on disabling errors.

(implicitly, I'm also using node 16/the new package-lock format).

### disabling rules and next steps

I've introduced several new disabled rules. Let me quickly explain what's going on; there are two categories of rules I've disabled:

1. rules that were temporary disables; they were frequent enough that I couldn't manually resolve them, but should be simple. **I plan on opening issues to re-enable each of these rules**, just after this PR
    - `declaration-block-no-redundant-longhand-properties`: this is just tedious and error-prone
    - `no-descending-specificity`: this one is tricky since it could have impacts on the cascade (though that seems unlikely)
    - `scss/no-global-function-names`: I think we need to [import map and then use `map.get`](https://stackoverflow.com/questions/64210390/sass-map-get-doesnt-work-map-get-does-what-gives), but I'll leave this as out of scope for now
2. rules that are long-term disables; due to the SASS-based nature of our theme, I think we'll keep these in limbo
    - `alpha-value-notation` causes problems with SASS using the `modern` syntax - literals like `50%` are not properly interpolated, and they cause formatting issues on the site
    - `color-function-notation` also causes problems with SASS, but in this case the `modern` syntax breaks SASS compilation; we're not alone (see this [SO post](https://stackoverflow.com/questions/71805735/error-function-rgb-is-missing-argument-green-in-sass)). 

In addition, we have many inline `stylelint-disable` comments. I'd open a separate issue to audit them, especially since I think some disables are unnecessary.

### on Primer 

**note: there hasn't been much other discussion, so I'm going to remove primer's stylelint config.**

If I do add `@primer/stylelint-config`, I get *a ton* of errors about now using `@primer`'s in-built SCSS variables. I imagine that we probably won't want to use these presets (though I could be wrong). In that case, I think we could either:

1. disable all of those rules
4. not use `@primer/stylelint-config`, since we're not actually using primer, and shift back to the standard SCSS config provided by Stylelint

~~Any thoughts here? I also don't have the original context as to why we do use the primer rules, perhaps @pmarsceill can chime in?~~

_sass/support/_functions.scss

@@ -1,9 +1,9 @@
 @function rem($size, $unit: "") {
-  $remSize: $size / $root-font-size;
+  $rem-size: $size / $root-font-size;
 
-  @if ($unit == false) {
-    @return #{$remSize};
+  @if $unit == false {
+    @return #{$rem-size};
   } @else {
-    @return #{$remSize}rem;
+    @return #{$rem-size}rem;
   }
 }

_sass/support/_variables.scss

@@ -1,82 +1,67 @@
-//
 // Typography
-//
 
-$body-font-family: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI",
-  Roboto, "Helvetica Neue", Arial, sans-serif !default;
-$mono-font-family: "SFMono-Regular", Menlo, Consolas, Monospace !default;
+$body-font-family: system-ui, -apple-system, blinkmacsystemfont, "Segoe UI",
+  roboto, "Helvetica Neue", arial, sans-serif !default;
+$mono-font-family: "SFMono-Regular", menlo, consolas, monospace !default;
 $root-font-size: 16px !default; // Base font-size for rems
 $body-line-height: 1.4 !default;
 $content-line-height: 1.6 !default;
 $body-heading-line-height: 1.25 !default;
 
-//
 // Font size
 // `-sm` suffix is the size at the small (and above) media query
-//
 
 $font-size-1: 9px !default;
 $font-size-1-sm: 10px !default;
-$font-size-2: 11px !default; //h4 - uppercased!, h6 not uppercased, text-small
-$font-size-3: 12px !default; //h5
+$font-size-2: 11px !default; // h4 - uppercased!, h6 not uppercased, text-small
+$font-size-3: 12px !default; // h5
 $font-size-4: 14px !default;
-$font-size-5: 16px !default; //h3
-$font-size-6: 18px !default; //h2
+$font-size-5: 16px !default; // h3
+$font-size-6: 18px !default; // h2
 $font-size-7: 24px !default;
-$font-size-8: 32px !default; //h1
+$font-size-8: 32px !default; // h1
 $font-size-9: 36px !default;
 $font-size-10: 42px !default;
 $font-size-10-sm: 48px !default;
 
-//
 // Colors
-//
 
 $white: #fff !default;
-
 $grey-dk-000: #959396 !default;
 $grey-dk-100: #5c5962 !default;
 $grey-dk-200: #44434d !default;
 $grey-dk-250: #302d36 !default;
 $grey-dk-300: #27262b !default;
-
 $grey-lt-000: #f5f6fa !default;
 $grey-lt-100: #eeebee !default;
 $grey-lt-200: #ecebed !default;
 $grey-lt-300: #e6e1e8 !default;
-
 $purple-000: #7253ed !default;
 $purple-100: #5e41d0 !default;
 $purple-200: #4e26af !default;
 $purple-300: #381885 !default;
-
 $blue-000: #2c84fa !default;
 $blue-100: #2869e6 !default;
 $blue-200: #264caf !default;
 $blue-300: #183385 !default;
-
 $green-000: #41d693 !default;
 $green-100: #11b584 !default;
 $green-200: #009c7b !default;
 $green-300: #026e57 !default;
-
 $yellow-000: #ffeb82 !default;
 $yellow-100: #fadf50 !default;
 $yellow-200: #f7d12e !default;
 $yellow-300: #e7af06 !default;
-
 $red-000: #f77e7e !default;
 $red-100: #f96e65 !default;
 $red-200: #e94c4c !default;
 $red-300: #dd2e2e !default;
-
 $body-background-color: $white !default;
 $sidebar-color: $grey-lt-000 !default;
 $search-background-color: $white !default;
 $table-background-color: $white !default;
 $code-background-color: $grey-lt-000 !default;
 $feedback-color: darken($sidebar-color, 3%) !default;
-
 $body-text-color: $grey-dk-100 !default;
 $body-heading-color: $grey-dk-300 !default;
 $search-result-preview-color: $grey-dk-000 !default;
@@ -85,9 +70,7 @@ $link-color: $purple-000 !default;
 $btn-primary-color: $purple-100 !default;
 $base-button-color: #f7f7f7 !default;
 
-//
 // Spacing
-//
 
 $spacing-unit: 1rem; // 1rem == 16px
 
@@ -104,7 +87,6 @@ $spacers: (
   sp-9: $spacing-unit * 3.5,
   sp-10: $spacing-unit * 4,
 ) !default;
-
 $sp-1: map-get($spacers, sp-1) !default; // 0.25 rem == 4px
 $sp-2: map-get($spacers, sp-2) !default; // 0.5  rem == 8px
 $sp-3: map-get($spacers, sp-3) !default; // 0.75 rem == 12px
@@ -116,17 +98,13 @@ $sp-8: map-get($spacers, sp-8) !default; // 3    rem == 48px
 $sp-9: map-get($spacers, sp-9) !default; // 3.5  rem == 56px
 $sp-10: map-get($spacers, sp-10) !default; // 4  rem == 64px
 
-//
 // Borders
-//
 
 $border: 1px solid !default;
 $border-radius: 4px !default;
 $border-color: $grey-lt-100 !default;
 
-//
 // Grid system
-//
 
 $gutter-spacing: $sp-6 !default;
 $gutter-spacing-sm: $sp-4 !default;
@@ -140,9 +118,7 @@ $header-height: 60px !default;
 $search-results-width: $content-width - $nav-width !default;
 $transition-duration: 400ms;
 
-//
 // Media queries in pixels
-//
 
 $media-queries: (
   xs: 320px,

_sass/support/mixins/_layout.scss

@@ -10,7 +10,7 @@
   $value: map-get($media-queries, $name);
 
   // If the key exists in the map
-  @if $value != null {
+  @if $value {
     // Prints a media query based on the value
     @media (min-width: rem($value)) {
       @content;