Migrate to Sass modules (#41512)

* Reorganize scss folder

* Migrate to Sass modules

* Migrate docs to Sass modules, comment out docs grid CSS

* Give helpers folder an index.scss, migrate ratio helper to aspect-ratio utility

* Delete node sass Action

* Modify Sass tests to pass for new Sass modules implementation

* Don't disallow calc()

* Move heading classes back to Reboot to prevent a dependency

* Utilities, some helpers, and theme colors

* Temporary fix of docs compilation

* Temporary Bundlewatch fix

* docs fix import to use

* Restyle docs callouts

* Fix docs colors

* Revert typo

* Reintroduce `css-lint-vars` npm script

* Bump to Sass v1.90.x

* Fixes

* more

* Remove

---------

Co-authored-by: Julien Déramond <[email protected]>

Author: mdo

.bundlewatch.config.json

@@ -10,27 +10,27 @@
     },
     {
       "path": "./dist/css/bootstrap-reboot.css",
-      "maxSize": "3.5 kB"
+      "maxSize": "4.25 kB"
     },
     {
       "path": "./dist/css/bootstrap-reboot.min.css",
-      "maxSize": "3.25 kB"
+      "maxSize": "4.25 kB"
     },
     {
       "path": "./dist/css/bootstrap-utilities.css",
-      "maxSize": "11.75 kB"
+      "maxSize": "12.75 kB"
     },
     {
       "path": "./dist/css/bootstrap-utilities.min.css",
-      "maxSize": "11.0 kB"
+      "maxSize": "12.0 kB"
     },
     {
       "path": "./dist/css/bootstrap.css",
-      "maxSize": "32.5 kB"
+      "maxSize": "33.75 kB"
     },
     {
       "path": "./dist/css/bootstrap.min.css",
-      "maxSize": "30.25 kB"
+      "maxSize": "31.25 kB"
     },
     {
       "path": "./dist/js/bootstrap.bundle.js",