Intent to prototype and ship: rch, rex, rcap, and ric CSS units

[Emilio Cobos Álvarez]

Summary: Few new CSS units for root-relative font metrics. Other
browsers already implemented these and they are a relatively
non-controversial extension to CSS.

The patch has been contributed by Sajid Anwar. Thank you!

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1740584
Specification: https://drafts.csswg.org/css-values/#font-relative-lengths
Platform coverage: All
Preference: none (relatively low risk patch)
DevTools bug: N/A, I think, should work the same way as existing CSS units.
Extensions bug: N/A
Link to standards-positions discussion:
https://github.com/mozilla/standards-positions/issues/725
Other browsers: Shipping in both Blink and WebKit
web-platform-tests: There's a variety of tests in css-fonts /
css-values, and we adjusted and added some more, see the patches for
details.

Thanks,
-- Emilio