Investigate problems with Twemoji Mozilla

Recently, we discovered that while we bundle Twemoji Mozilla on Windows, it isn't loaded, because we don't list it in font.system.whitelist.

We should investigate whether this is an upstream bug, like Bugzilla 1745715, or whether we should add the font manually to that list.

On Mac, we don't ship it (@duncan couldn't set it as a font, and I've checked in the .dmg, and it isn't there), and we possibly should, for #40860 (closed).