Upgrade to Stylelint 15 (#1185)

This is an internal change that only affects developers of our theme. This PR:

- **upgrades `Stylelint` to `v15`!**
    -  among other things, this leads to the deprecation of many stylistic rules, which overlap with Prettier
    - it also sees the addition of new rules that catch more errors, especially `declaration-property-value-no-unknown`!
- upgrades the accompanying standard config (the SCSS one extends the first-party one); this new config disables the deprecated rules that overlap with Prettier
- removes a now-unneeded plugin that disables overlapping rules
- moves the `stylelint` config object to `package.json` - now we have one less file (particularly important since people still clone this repo)

There were very few new changes; I combined one string that had a dangling `+` in an SCSS warning, and have temporarily disabled the `at-rule-empty-line-before` rule (which also picks up on SCSS `@includes`).

_sass/support/mixins/_layout.scss

@@ -16,8 +16,7 @@
       @content;
     }
   } @else {
-    @warn "No value could be retrieved from `#{$media-query}`. "
-      + "Please make sure it is defined in `$media-queries` map.";
+    @warn "No value could be retrieved from `#{$media-query}`. Please make sure it is defined in `$media-queries` map.";
   }
 }