[css-nesting] Implement CSS OM read-only (serialize CSS rule) with nesting

[Matthieu Dubet]

https://drafts.csswg.org/cssom/#serialize-a-css-rule

The serialization of "implicit" nested rule inside group rule such as:

 #foobar { @media { color:black; } } 

which is implicitly

#foobar { @media { & { color:black; } } }

is not precisely specified, but we should try to serialize the minimal equivalent

[Radar WebKit Bug Importer]

<rdar://problem/107190613>

[Matthieu Dubet]

Pull request: https://github.com/WebKit/WebKit/pull/12008

[EWS]

Committed 262177@main (f4dfe341bdcc): <https://commits.webkit.org/262177@main>

Reviewed commits have been landed. Closing PR #12008 and removing active labels.