Editor: code quality improvements for theme.json migrate API

[oandregal]

Trac ticket https://core.trac.wordpress.org/ticket/61282

Commit message:

Editor: code quality improvements for theme.json migrate API

Backports https://github.com/WordPress/gutenberg/pull/62305

Follow-up to [58328], #61282.

Props ajlende, oandregal, ramonopoly, mukesh27.
Fixes #61282.

[github-actions]

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Props oandregal, ramonopoly, mukesh27.

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[oandregal]

cc @ajlende @ramonjd

[github-actions]

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[ramonjd]

I tested the functionality introduced in

• Backport theme.json version 3 migrations #6616

Also smoke tested user styles in site editor/updating theme.json values to make sure they're reflected in the CSS output.

Checked that the changes match WordPress/gutenberg#62305

👍🏻

[oandregal]

Committed in https://core.trac.wordpress.org/changeset/58354