This type is a lot of complexity related to a very specific thing such as the
hover and active quirk.
Instead of that, move `nesting_level` to `MatchingContext`, and simplify all
this computing whether the quirk applies upfront, for each complex selector we
test.
This is less error-prone, and also allows simplifying more stuff in a bit.
Adjust the matching process to look for a "relevant link" while matching. A
"relevant link" is the element being matched if it is a link or the nearest
ancestor link.
Matching for links now depends on the `VisitedHandlingMode`, which determines
whether all links match as if they are unvisited (the default) or if the
relevant link matches as visited (and all others remain unvisited).
If a relevant link is ever found for any selector, track this as part of the
`MatchingContext` object. This is used in the next patch to determine if an
additional match and cascade should be performed to compute the styles when
visited.
MozReview-Commit-ID: 3xUbRo7vpuD
