Theme JSON: Add missing i18n for variations

[aaronrobertshaw]

This PR backports WordPress/gutenberg#62552

The changes included add support for internationalizing the block style variations labels defined from a theme.json or theme style variation file.

Test Instructions

In the theme.json of the theme, paste the following contents under styles.blocks.variations:

"myvariation": {
    "title": "My Variation",
    "blockTypes": ["core/group"],
    "color": {
        "background": "var(--wp--preset--color--base-2)"
    }
}

Without the title	With the title

Trac ticket: https://core.trac.wordpress.org/ticket/61442

---

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, oandregal.

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[oandregal]

This PR also unblocks changes at the i18n-command at wp-cli/i18n-command#405 that depend on this schema change.

[github-actions]

Test using WordPress Playground

The changes in this pull request can previewed and tested using a WordPress Playground instance.

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Some things to be aware of

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For more details about these limitations and more, check out the Limitations page in the WordPress Playground documentation.

Test this pull request with WordPress Playground.

[oandregal]

Committed at https://core.trac.wordpress.org/changeset/58412