Implement CSS filter for Stylo
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This patch is part of https://bugzilla.mozilla.org/show_bug.cgi?id=1310117
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It is called nsstring_vendor since libgkrust already depends on nsstring, and
we can't have two crates with the same name in-tree. We can't directly depend
on the in-tree one since this will stop independent `./mach build-geckolib` builds from working.
MozReview-Commit-ID: CR4tHgzXWFZ
