Add support for gecko-only array values; use for stylo
Doesn't yet work, because I can't grow nsTArrays from Rust. If anyone knows how to add bindings for that, let me know!
Thoughts on the design so far? Once this PR lands, all of the array-accepting background- properties can just use gecko_autoarray_longhand and some iterators in the geckolib implementation without changing much other code to work with arrays.
cc @emilio @bholley
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