Add Style Engine support for nested CSS rules.

[tellthemachines]

Trac ticket: https://core.trac.wordpress.org/ticket/61099

Syncs the changes from WordPress/gutenberg#58867 and WordPress/gutenberg#58922.

This isn't really testable without a code change; you can check out usage in this PR (which will be also synced to Core in this release cycle).

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[github-actions]

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[tellthemachines]

Tests added, marking this ready for review!

[github-actions]

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Props isabel_brison, ramonopoly.

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[ramonjd]

Thanks for porting over these changes. LGTM

Ran the unit tests and also created some grouped styles manually using wp_style_engine_get_stylesheet_from_css_rules - results were as expected.

🍺

[ramonjd]

Should new methods and updates to other public methods/APIs get a @since annotation?

[tellthemachines]

Oh good point, yeah they should!

[tellthemachines]

Updated ✅

[tellthemachines]

Committed in r58089.