fix: allow for nesting selector in pseudoclasses

[paoloricciuti]

Svelte 5 rewrite

Closes #13203

The problem is that we were checking for NestingSelectors only in the selectors of the immediate child of the selectors. Since Nesting could be in selectors like :is or :where or :has that can also be nested i think we need to walk the selectors to find if there's a selector or not.

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