WP_Theme: Add "has_theme_json" method

[wildworks]

We plan to expose the has_theme_json field in the Theme Rest API in #63253. However, because the wp_theme_has_theme_json() function is based on the active theme, the has_theme_json field is not correct when fetching the theme collection with the following code, for example:

wp.data.select( 'core' ).getEntityRecords( 'root', 'theme', { per_page: -1, _fields: [ 'stylesheet', 'has_theme_json', 'is_block_theme' ] } );

To solve this, I think we need a has_theme_json() method, similar to the block_theme() method.

Trac ticket: https://core.trac.wordpress.org/ticket/63299

[wildworks]

I'm not sure how to apply this PR to Gutenberg because WP_Theme is a final class and the Gutenberg plugin cannot extend it.