This changeset adds a `is_block_theme` property to each theme in the `wp/v2/themes` API response, which uses `WP_Theme::is_block_theme` to determinate whether
the theme is block theme or not.
Props grantmkin, ironprogrammer, zunaid321, azaozz, spacedmonkey, audrasjb, costdev.
Fixes#58123.
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