Revamp the popover target attributes

Instead of popoverhidetarget, popovershowtarget, and popovertoggletarget, we will now have popovertarget and popovertargetaction. The former for targeting an element and the latter for determining what to do with the target.

This also corrects popover validity checks in activation behavior and consistifies the popover attribute.

Tests: web-platform-tests/wpt#38701.

HTML-AAM: w3c/html-aam#446.

Fixes #8894, fixes #8983, and fixes #8979.