Add bash script for linting variables, remove unused ones

[mdo]

This PR adds the script from @badsyntax at https://gist.github.com/badsyntax/6193491 to our build directory so we can easily lint our variables. A more ideal solution would be an scss-lint task, but the processes there seem more complex and less commonly used. This was the first thing that popped up that worked well and was easy to setup :).

During testing, found that I left the button and badge variants in there, as well as some other small variables from cards, forms, etc.

Fixes #18623.