Auto merge of #10934 - emilio:other-gecko-pseudos, r=bholley,mbrubeck

style: Add infrastructure to support lazy pseudo-elements

This builds on top of #10815, so it's really just the last commit the one that should be reviewed.

I tried to apply the new infrastructure to servo, but failed (for now?).

The problem with it is that it'd require `ThreadSafeLayoutElement` to implement `selectors::Element`, which is a lot of work and might be racy (not totally sure about it though). Thus, I prefered to keep selectors eager until knowing that it's safe to do it.

r? @mbrubeck for style changes, @bholley for the geckolib changes (minimal for now, glue + a list of lazy PEs must be added)

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