The other part of #13395
This implements scoping for selectors inside `:not(...)`. The approach is almot the same as for `:is/where(...)`.
This is a breaking change because people could've used `:not(.unknown)` with `.unknown` not appearing in the HTML, and so they need to do `:not(:global(.unknown))` instead.
While implementing it I also discovered a few bugs, which are fixed in this PR:
- `foo :is(bar baz)` wasn't properly handled. This selector can mean `foo bar baz` but it can also mean `bar foo baz` (super weird, but it is what it is). Since our current algorithm isn't suited for handling this, we just assume it matches and scope it. Worst case is we missed a prune
- `bar` in `:global(foo):is(bar)` was always marked as unused, even if it matched