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