Previously, and according to the spec, `a::part(foo)::before` would be a
single CompoundSelector, even though it matches against 3 different
targets. This meant some awkward swapping of targets in the middle of
matching, and in particular it made `::part()` and `::slotted()` quite
hacky, requiring them to track extra data on the MatchContext to then
use later. This was scattered around and difficult to follow.
Partly inspired by Gecko, this commit instead introduces an invisible
PseudoElement combinator. After parsing a selector, we find any
CompoundSelectors that contain a pseudo-element and split them up, so
that each CompoundSelector only has a single target in the end. Where
the pseudo-element was at the start of a CompoundSelector, we insert an
invisible universal selector before it to represent its originating
element.
So now, a CompoundSelector deals with one target, and switching targets
is done at the combinator.
The one inconsistency is that we match the target of ::slotted()
and ::part() in pseudo_element_transition_target(), instead of before
then when processing the SimpleSelector. This is to avoid repeating the
same computations twice.
No outward-facing behaviour changes, though the invalidation metrics
have changed.
Document.cpp contained the CSS rule-cache matching used to decide which
elements need style updates when hover, focus, or target state changes.
Move that logic into CSS::Invalidation::PseudoClassInvalidator.
Document still owns the current state slots and chooses when a state
transition happens. The helper now owns the selector matching and
recursive invalidation pass for those pseudo-class transitions.
