theme.json: add title key to i18n schema

[oandregal]

Follow-up to #39322 and #39715
Related WordPress trunk PR at WordPress/wordpress-develop#2483

What?

This PR adds title as a new key to translate from a theme.json file. It's necessary to fix the wp-cli i18n-command tests at wp-cli/i18n-command#306

Why?

The title key from a style variation is shown to the user, hence, it needs to be translatable.

How?

By adding the key to the i18n schema we allow the i18n-command from the wp-cli to pick it up. See wp-cli/i18n-command#306 Though the wp-cli uses the schema from WordPress trunk so we also need to port the changes there.

Testing Instructions

This PR only prepares the code for the wp-cli to pick up translations, so there's nothing to test other than verifying that it doesn't break anything. This is what I did:

• Go to "Settings > General" and set the language to Spanish.
• Use TwentyTwentyTwo.
• Load the site editor.
• Verify that the color palettes names show up in Spanish (go to "Global Styles > Colors > Palette" and hover over the color circles.

[youknowriad]

Something I was wondering, we probably need to wrap the "title" property in __ somewhere around here right?

gutenberg/lib/compat/wordpress-6.0/class-gutenberg-rest-global-styles-controller.php

Lines 234 to 238 in 2a2938f

	$variation = ( new WP_Theme_JSON_Gutenberg( $decoded_file ) )->get_raw_data();
	if ( empty( $variation['title'] ) ) {
	$variation['title'] = basename( $path, '.json' );
	}
	$variations[] = $variation;

[oandregal]

Something I was wondering, we probably need to wrap the "title" property in __ somewhere around here right?

Yeah, we need to do other things for it to be picked up. I've already started looking into this, but I also want to fast-track the schema update and the corresponding WordPress trunk PR to unblock the i18n-command PR wp-cli/i18n-command#306

[mcsf]

Everything is 👌.