feat: CSS language plugin

[nzakas]

This is the first commit for the CSS language plugin. It implements the basic functionality along with the general infrastructure necessary for the plugin.

I've included just one rule to ensure that the language plugin works. We can add additional rules on a one-off basis after this initial PR is merged.

[nzakas]

One thing I've noticed about this parser: it is very fault-tolerant, just like the CSS parser built into the browser. That means something like a { is parsed fine without an errors and produces a valid AST even though the syntax is not correct. 🤔

csstree/csstree#301

[romainmenke]

a { is actually correct syntax.

https://drafts.csswg.org/css-syntax/#consume-simple-block

<eof-token>
ending token
Discard a token from input. Return block.

Encountering an <eof-token> is handled the same as encountering the ending token for a block.

[nzakas]

@romainmenke it may be correct syntax by the spec, but users would expect a linter to pick that up.

[romainmenke]

Sure, no disagreement there :)

[nzakas]

I've updated to catch the errors that csstree does report. Waiting to hear back on a {.

[nzakas]

CSSTree updated Blocks so that curly braces are now included in the location, meaning I could remove the extra CSSSourceCode methods for updating Block locations. 🎉

[mdjermanovic]

Last few suggestions, then LGTM.

[mdjermanovic]

Is it guaranteed that all array items are always AST nodes, not e.g., null as can be the case in ESTree (for example, in ArrayExpression#elements)?

[nzakas]

As far as I can tell, yes.

[mdjermanovic]

LGTM, thanks!

I'm not sure if the a { parsing is a blocker for merging this and releasing the first version of this plugin. If it isn't, I think this is ready to merge & release.

[nzakas]

It shouldn't be a blocker. I just left it in there to remind me to follow up.