Global Styles: Update util for scoping CSS selectors

[aaronrobertshaw]

Syncs the changes from:

• Global Styles: Update utils for scoping CSS selectors gutenberg#61026

Adds early return for existing scope_selector method making it more robust and ready for extending block style variations. See: WordPress/gutenberg#57908

Unit tests: npm run test:php -- --filter Tests_Theme_wpThemeJson

Trac ticket: https://core.trac.wordpress.org/ticket/61120

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[aaronrobertshaw]

Thanks for fixing up the docblock comment @audrasjb 🚀

[tellthemachines]

LGTM!

[tellthemachines]

Would be nice if there were a test for this, but given that it's an existing function I think it's fine to do that separately (and tests can be added anytime during the release cycle).

[tellthemachines]

Committed in r58245.