Global Styles: Avoid unnecessary processing of theme.json variation partials

[aaronrobertshaw]

This PR backports the PHP updates from:

• Global Styles: Skip registration of variation styles when unsupported gutenberg#62529

Trac ticket: https://core.trac.wordpress.org/ticket/61451

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[github-actions]

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[github-actions]

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Props aaronrobertshaw, oandregal, mukesh27.

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[oandregal]

I've scoped down this backport to only the changes in WordPress/gutenberg#62529 The other PR needs more time to clarify questions/address feedback.

[oandregal]

Committed at https://core.trac.wordpress.org/changeset/58423