Add all CSS length units in known compatibilities

[stof]

This allows more validation of wrong computations in first-class calc.

This adds the missing font-relative and viewport-relative units from https://drafts.csswg.org/css-values-4/#font-relative-lengths

[nex3]

Which of these are actually implemented in a real browser? That's our metric for whether to add support for a new feature in the spec.

[stof]

Based on https://developer.mozilla.org/en-US/docs/Web/CSS/length#browser_compatibility, only rlh is not yet implemented (but based on https://bugs.webkit.org/show_bug.cgi?id=246522, it is implemented in Safari TP so will probably be there in the next Safari release). And cap is implemented only in Firefox for now.
And looking at that list, I forgot the container query units (which are defined in a different CSS module but have cross-browser support)

[stof]

And https://bugs.chromium.org/p/chromium/issues/detail?id=1381037 marks rlh (and a few other r* units) as implemented in Chromium 2 days ago.

[nex3]

Sounds good. Can you add a changelog entry as well?

[stof]

@nex3 done.