fix: ensure :has selectors followed by other selectors match

[dummdidumm]

I resisted this previously because it felt a bit wasteful, but I now think that there's really no way around this: Instead of only going upwards the tree while matching, for :has we go down the tree to see what matches. More specifically, we're collecting the children of the current element and then check if one of those does match the selectors inside :has.

This makes the way the code works easier to reason about and also removes some boolean tracking we had to add for the previous approach.

Fixes #13779

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