Bug 1819711 - Enable :nth-child(... of <selector>) on Nightly. r=zrhoffman

This makes it easier to track where our implementation stands in
https://wpt.fyi vs. others.

The big piece of functionality missing is invalidation for some kinds of
DOM mutations (most notably, not for child changes), see bug 1809754 and
dependencies.

Differential Revision: https://phabricator.services.mozilla.com/D171376