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/color_schemes/dark.scss

@@ -1,28 +1,24 @@
 $body-background-color: $grey-dk-300;
 $sidebar-color: $grey-dk-300;
 $border-color: $grey-dk-200;
-
 $body-text-color: $grey-lt-300;
 $body-heading-color: $grey-lt-000;
 $nav-child-link-color: $grey-dk-000;
 $search-result-preview-color: $grey-dk-000;
-
 $link-color: $blue-000;
 $btn-primary-color: $blue-200;
 $base-button-color: $grey-dk-250;
-
 $search-background-color: $grey-dk-250;
 $table-background-color: $grey-dk-250;
 $feedback-color: darken($sidebar-color, 3%);
 
-//
 // The following highlight theme is more legible than that used for the light color scheme
-//
 
 // @import "./vendor/OneDarkJekyll/syntax-one-dark";
 // $code-background-color: #282c34;
 
 @import "./vendor/OneDarkJekyll/syntax-one-dark-vivid";
+
 $code-background-color: #31343f;
 
 // @import "./vendor/OneDarkJekyll/syntax-firewatch";