(feat) Restorer: investigate allowing child removal to trigger focus restoration in Safari & Firefox

[smhigley]

Related to #305 -- with the fix in #306, Restorer will work in Safari & Firefox if the root node itself is removed from the DOM, but not if a child node containing document.activeElement is removed (note - CSS changes like display: none will still work, only DOM removal is an issue).

This seems like more of a minor use case/feature to support, since the primary use case is likely removing the root node. This issue is for tracking & to not lose the edge case after #306 is closed. Discussed w/ @ling1726

[ling1726]

Using focusout to detect child removal will still be supported for chromium based browsers - no change there