Auto merge of #25424 - pshaughn:labelfixes, r=Manishearth

Make labelable element .labels a live list in tree order

This is not the highest-performance solution possible but it's visibly spec-aligned in a way a faster-performing implementation would be harder to verify, and I don't expect label-getting to deal with more than a few nodes at once in practice.
I added a macro by analogy with some of the existing make_XXX_getter! macros; I will change it if it doesn't seem right.
Remaining test failures are because keygen, shadow DOM, and ElementInternals are unimplemented. Shadow DOM should already be handled by the existing code when it is implemented, and keygen should just be able to add a labels_node_list and use the macro like the other labelable elements. ElementInternals labels are slightly different and might need another NodeList case.

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