Theme REST API: expose "has_theme_json" field

[wildworks]

Currently, there is no official way to detect whether the active theme has theme.json or not via REST API.

We needed to check for the existence of theme.json to conditionally expose Stylebook for the classic theme. As a temporary approach, we used the supportsLayout setting, which is the equivalent of the wp_theme_has_theme_json() function:

​https://github.com/WordPress/gutenberg/blob/a4a5b07b4432d28ad4eb3646435bbaf9a8f9796e/packages/edit-site/src/components/site-editor-routes/utils.js#L11-L12

However, there is no guarantee that this setting will remain equivalent to wp_theme_has_theme_json() in the future.

We should expose the new "has_theme_json" field via the theme REST API.

Trac ticket: https://core.trac.wordpress.org/ticket/63253

Gutenberg PR: ​https://github.com/WordPress/gutenberg/pull/69857

[wildworks]

Testing Instructions

• Open the post editor or the site editor
• Open the console panel of your browser
• Run wp.data.select('core').getCurrentTheme()
• Confirm that the response includes the has_theme_json field

Hey @t-hamano,

I have tested the PR, and it works well and adds the has_theme_json field in the themes REST API

## Screenshot:

#### Trunk:

#### This PR with no theme.json:

#### This PR with theme.json:

[wildworks]

Update: I realized that using wp_theme_has_theme_json() in the REST API response isn't correct. Because the wp_theme_has_theme_json() function returns results based on the active theme.

See ​https://github.com/WordPress/gutenberg/pull/69857#issuecomment-2801814369 for more details.

I think we need to add a new has_theme_json method to the WP_Theme class first, so I submitted #63299.