Translate global style variations

[oandregal]

WordPress 6.0 is adding the concept of style variations to Global Styles. These are separate theme.json files that provide a different set of styles.

There're a few things we need to do to be able to translate what they provide:

• make the title key from any theme.json translatable
  • requires changes to wp-cli i18n command
  • update the i18n schema available in core
• make wp-cli's i18n-command look for theme.json files under the styles/ folder of a theme (in addition to what it now does of looking in the top-level folder)

Trac ticket https://core.trac.wordpress.org/ticket/55495
This PR ports the related Gutenberg PR at ​https://github.com/WordPress/gutenberg/pull/39936

## 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 ​https://github.com/wp-cli/i18n-command/pull/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 ​https://github.com/wp-cli/i18n-command/pull/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.

Committed with ​https://github.com/WordPress/wordpress-develop/commit/39a5bacbc1a3a1d6cfd535995d330094b4ad22f0.

[gziolo]

Committed with [53038].