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

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Co-authored-by: Julien Déramond <juderamond@gmail.com>