Theme JSON: Extract util to get valid block style variations

[aaronrobertshaw]

Syncs the changes from:

• Theme JSON: Extract util to get valid block style variations gutenberg#61030

Extracts the repeated collection of valid block style variations into a util that can be reused or updated in a single place for future work around 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/61121

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