Add CSS (JS?) code formatting automation

Description of the need

It'd be nice to add an automation (GitHub Action) that formats code nicely for us in PRs. This'd not only make sure code is easy-to-read and consistent, but would prevent the back-and-forth, nit-picky reviews where we ask contributors to add some whitespace here, fix this comment to not overflow 80 characters, etc.

Proposed solution

As discussed in https://github.com/backdrop/backdrop-issues/issues/2785#issuecomment-108..., code linters seem to be moving away from 'stylistic' changes and just focusing on flagging/fixing bugs in code. In other words, a linter will warn you about a missing parentheses, but it won't flag whitespace or line-length issues.

This is where code formatters (or 'prettifiers') come in. I recommend we decide on which we'd like to use and then set it up as a GH action to run on PRs.

Alternatives that have been considered

We can always stick with our current system of having a documented coding style, and pointing contributors to that when we mark their PRs as 'needs work', but I don't think that's an ideal solution for reasons I'll get into below...

Draft of feature description for Press Release (1 paragraph at most)

Backdrop now automatically flags and/or fixes code formatting issues in pull requests for readability and consistency.

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Related: * #5245 * #2785