[Segment Cache] Prioritize hovered links (#74672)
This adds a new internal priority level to the prefetch queue for links
that are hovered or touched. The idea is that if a user hovers over a
link, they're much more likely to click on it.
The elevated priority is added on mouseenter/touchstart. It is _not_
removed on mouseleave/touchend, because even though the user left the
link without navigating to it, the link is still more likely to be
navigated to than a link that was never hovered at all.
Because the prefetch queue is last-in-first-out, the highest priority
link is whichever link was most recently hovered.
