[css-nesting] Support properties directly on nested conditional rules

Since there is no CSSOM defined for these yet, we parse them using an
implicit & {} rule. The spec should ideally also move in the same
direction; see https://github.com/w3c/csswg-drafts/issues/7850.

This completes CSS Nesting feature-wise as the spec currently stands,
with the caveat that we don't serialize @nest rules correctly
and do not have correct error recovery in all cases.

Bug: 1095675
Change-Id: I1dbef377885c23a2eaac82fcd7cf29d5d1b3919d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3967607
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Commit-Queue: Steinar H Gunderson <sesse@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1062133}