[cssom-view] Possible extensions to elementFromPoint

[emilio]

While going through #10992, I realized that Gecko at least has a fair amount of extra extensions for this:

• The ability of querying for nodes, not just Elements. That's useful with Shadow DOM specifically, because if you just have slotted text you have no way of getting it?
• The ability of hit testing a whole rect, not just a point. I'm pretty sure other engines need to have that capability as well for IntersectionObserver v2 and "fat finger" targetting.

These seems like logical / useful extensions to this (something like nodesFromRect(...)?).

[keithamus]

maybe dupe of #10398? At least related.