[css-contain] Move inline-size containment to the Level 2 spec

[sideshowbarker]

https://wpt.fyi/results/css/css-contain mostly shows green for all engines for the variouscontain-inline-* tests (though Safari seems to be failing 100% of the https://wpt.fyi/results/css/css-contain/contain-inline-size-replaced.html tests…)

That’d seem to suggest the “inline-size containment” feature is sufficiently mature enough to move into the Level 2 spec.

[w3cbot]

sideshowbarker marked as non substantive for IPR from ash-nazg.

[dbaron]

This feature is closely tied to much of the other material in contain level 3 -- it's an important prerequisite for container queries. Given that container query implementation is happening (I think) in multiple engines, I think this may be a somewhat temporary situation, and I suspect that many other features in contain-3 are likely to show similar level of stability in the near future.

[sideshowbarker]

Going ahead and closing this per https://github.com/w3c/csswg-drafts/pull/7631#issuecomment-1222442740