CSS parser

[Arnau478]

This PR introduces a CSS parser, as discussed in #4

State

The current state of the parser is the following:

• [x] Core CSS grammar
• [x] Formatter
• [x] Simple selectors (class, id, etc.)
• [x] Pseudo-classes and pseudo-elements
• [x] Multi-expression declarations (e.g. padding: 4px 2px;)
• [x] Functions (e.g. rgb(105,12,52))
• [x] @media
• [ ] @import, @font-face, @keyframes, etc.
• [ ] Selector combinators (>, +, etc.)
• [ ] Attribute selectors (e.g. a[href="..."])
• [ ] Full parse error coverage
• [ ] Crash stability (still has many @panic("TODO"))

TLDR: It can handle a decent amount of real-world stylesheets.

Overall, I would say it's advanced enough to have it in the main repo (although not wired in yet). Also, the fact that I'm opening a PR doesn't mean that it has to be merged right away, it means that from my side it's ready. Feel free to wait a bit before merging if you feel like doing so. I'll continue to work on my own fork until then.

Note

I'm not sure about what branch to target. GitHub lets me change it after opening the PR, so I've left it as the default (main) for now.

[kristoff-it]

I've invited you to the repo, when you accept the invite please merge your PR as your first official act :^)

I would recommend creating a merge commit in this case.