fix: better scope :global() with nesting selector &

[7nik]

Fixes #14616

Personal though: I think RelativeSelector with :global() must contain nothing else (exceptions are :not() and :has()) because it is ambiguous whether the part outside :global() should be scoped, and if yes, what is the point of :global() then. But it will be a breaking change.

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[Rich-Harris]

thanks!