Backport: Global Styles: Add scoping of feature level selectors

[aaronrobertshaw]

Syncs the changes from:

• https://github.com/WordPress/gutenberg/pull/61033

These changes ensure that custom feature-level selectors for style nodes are correctly scoped when generating a theme.json stylesheet

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

Trac ticket: https://core.trac.wordpress.org/ticket/61119

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[vcanales]

👋 @audrasjb @aaronrobertshaw any updates here?

[aaronrobertshaw]

All the feedback to date has been addressed and we're just waiting on an approval.

[tellthemachines]

Committed in r58244.