[css-pseudo] allowing ::search-text:current its own overlay

[delan]

#10212 resolved that ::search-text (#3812) will be a single highlight pseudo with two states, which currently implies a single highlight overlay (css-pseudo #highlight-bounds).

But while working on the ::search-text paint impl in Chromium, we found that splitting :current and :not(:current) into separate highlight overlays for paint makes the impl a lot less complicated. With a single overlay, the highlight overlay code needs to explicitly track where ::search-text was :current, so we can store and switch between two sets of computed and resolved styles for ::search-text (and ::selection). With two overlays, it’s all encoded in which overlay we choose.

We propose allowing impls to paint ::search-text:current and ::search-text:not(:current) in separate highlight overlays. This would have minor painting order side effects, like backgrounds of :current overlapping text shadows of :not(:current), but the rendering would otherwise be roughly the same.