Implement Highlight Pseudo-elements (::selection)

[paulrouget]

I'm not sure we can select text yet in a regular element, but it's possible in an <input> tag.The selection is not highlighted though.

See: https://drafts.csswg.org/css-pseudo-4/#selectordef-selection

[nox]

This will require changes in rust-selectors.

[paulrouget]

Bumping to bhtml P2. Now we have an awesomebar, we need to be able to highlight the selected text in an <input>.

[mbrubeck]

See also #4410 and #4409.

[paulrouget]

@SimonSapin - can you confirm you're working on this issue?

[SimonSapin]

@paulrouget As we’ve discussed in person I think #9993 can be separated from styling support. I haven’t started on this yet as I’d like to wrap up servo/rust-url#176 and its fallout first.

With a recent change to rust-selectors, parsing a new pseudo-element can be done in components/style/selector_impl.rs without changing rust-selectors.

[mbrubeck]

Fixed by #10252.