Intent to ship: break-inside: avoid-{page,column}

[Emilio Cobos Álvarez]

Summary: Allow authors to avoid breaks within an element differentiating
between column and page breks.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1722945

Standard: https://drafts.csswg.org/css-break/#break-within

Platform coverage: all

Preference: none, this is a small extension to the existing
break-inside: avoid support.

Devtools bug: N/A

Other browsers: Both WebKit and Blink support this since forever.

web-platform-tests: There are some basic tests for parsing /
serialization that we started passing, and a couple reftests. General
break-avoiding behavior is also well tested by existing `break-inside:
avoid` / `page-break-inside: avoid`.

-- Emilio