The main part of #13395
This implements scoping for selectors inside `:has(...)`. The approach is to first descend into the contents of a `:has(...)` selector, then in case of a match, try to match the rest of the selector ignoring the `:has(...)` part. In other words, `.x:has(y)` is essentially treated as `x y` with `y` being matched first, then walking up the selector chain taking into account combinators.
This is a breaking change because people could've used `:has(.unknown)` with `.unknown` not appearing in the HTML, and so they need to do `:has(:global(.unknown))` instead
Simon H