WP_Theme_JSON: Add nested indexed array schema sanitization capabilities.

[matiasbenedetto]

What?

• Add nested indexed array schema sanitization capabilities to the WP_Theme_JSON class.
• Add settings.typography.fontFamilies validation in WP_Theme_JSON
• Add unit tests.

Why?

Currently, WP_Theme_JSON sanitization is not able to sanitize data contained on indexed arrays. So certain data from theme.json, for example, settings.typography.fontFamilies which is a JSON array, cannot be sanitized. Why? because when parsing the JSON data WP_Theme_JSON class translates JSON arrays into PHP indexed arrays and the it is not capable of sanitizing that kind of data.

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Trac ticket: https://core.trac.wordpress.org/ticket/60360#

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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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[mukeshpanchal27]

Fix PHPCS

[youknowriad]

Commit https://core.trac.wordpress.org/changeset/57496