Add the beginning of hover-trigger for pop-ups

This CL adds the 'hoverpopup' attribute, to go along with the other
invoking attributes (togglepopup, showpopup, hidepopup). This attribute,
when places on *any* element, triggers a pop-up when hovered. With this
CL, it is always triggered after a fixed 100ms delay, but that will be
made configurable via CSS in subsequent CLs. There is also no "hide"
trigger when any element is de-hovered - again that will be added
later.

See this spec discussion:
 https://github.com/openui/open-ui/issues/526#issuecomment-1161917067

Bug: 1307772
Change-Id: I7af88dad9efa015f833843ea76bed41b4aa42c4b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3759184
Reviewed-by: David Baron <dbaron@chromium.org>
Auto-Submit: Mason Freed <masonf@chromium.org>
Commit-Queue: Mason Freed <masonf@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1024141}