CI: Check block.json and theme.json according to schemas

[sirreal]

What?

Add JSON schema checking to CI. This should help prevent issues appearing in schemas in the repo.

This has #71852 merged to fix existing issues.

Closes #71817.

Why?

It's good to prevent schema errors in the files in the repo. See #71800 for an example.

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Co-authored-by: sirreal <jonsurrell@git.wordpress.org>
Co-authored-by: t-hamano <wildworks@git.wordpress.org>

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[t-hamano]

Thanks for the PR.

How about adding unit tests instead of CI? This is because there are already unit tests that check the theme.json and wp-env.json schemas.

• https://github.com/WordPress/gutenberg/blob/a4988ec5a6c50b8a851e5f156280f14610004f50/test/integration/theme-schema.test.js
• https://github.com/WordPress/gutenberg/blob/a4988ec5a6c50b8a851e5f156280f14610004f50/test/integration/wp-env-schema.test.js

[sirreal]

👌 I didn't realize there were already unit tests for JSON schema validation.