chore: use zimmerframe for CSS analysis/transformation

[Rich-Harris]

Right now, CSS handling lives in its own universe — we parse the <style> tag (sloppily), then bludgeon it into shape with classes defined in Stylesheet.js and Selector.js that both use zimmerframe and reimplement its traversal behaviour. This takes more code and more CPU cycles than just using the same techniques we use everywhere else for analysis and traversal.

As I started working on :is(...) support I realised it's also a fairly inflexible approach. I therefore intend to remove a lot of the code in Stylesheet.js and Selector.js in favour of something more harmonious. Right now it looks like more code, but I'm reasonably confident it will end up being significantly less once it's done.

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Latest commit: 0e7fc47

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[Rich-Harris]

(note: the diff is more green than red, but that's because of the tests — the AST now contains more granular detail, which means there's a bunch of bigger output.json files)

[trueadm]

LGTM

[AlbertMarashi]

@Rich-Harris can you link relevant PRs for the CSS nesting you might create so I can follow along