Auto merge of #12815 - emilio:stylo-atoms, r=bholley

stylo: Use atoms as the pseudo-element back-end.

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A bit of work left, and we can uber-optimize this (left comments, will fill follow-ups), but this is a decent refactor so I thought I'd rather get some feedback on it.

r? @bholley (not formally yet, maybe, but some feedback is appreciated).

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- [x] These changes do not require tests because geckolib only...

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