Block Styles: Fix block style variation selector generation

[aaronrobertshaw]

Syncs the changes from WordPress/gutenberg#58051.

These changes fix the generation of selectors for block style variations. Previously, an incorrect CSS selector could be generated if the block's base selector used an element tag etc.

To test:

Run: npm run test:php -- --filter Tests_Theme_wpThemeJson

If you want to go further and test manually:

1. Add a block style to the paragraph block in its block.json file
2. Within Global Styles, set a text color or something for the block style variation you just added
3. Apply the variation to a block instance and view it on the frontend
4. Inspect paragraph and confirm the variation's selector e.g. is-style-dark is appended to the p tag rather than prefixing it

Trac ticket: https://core.trac.wordpress.org/ticket/60453

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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Props aaronrobertshaw, youknowriad, mukesh27.

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Co-authored-by: aaronrobertshaw <aaronrobertshaw@git.wordpress.org>
Co-authored-by: youknowriad <youknowriad@git.wordpress.org>
Co-authored-by: mukeshpanchal27 <mukesh27@git.wordpress.org>

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[github-actions]

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[youknowriad]

Thanks for the comprehensive tests.

[youknowriad]

commit https://core.trac.wordpress.org/changeset/57547