When a theme that uses the Full Site Editing feature is activated and the Gutenberg plugin is not present, the site will currently show a text notice on the front end. The user is not made aware of this unless they visit the front end of their site.
This adds a check that will prevent a theme from being activated when the `full-site-editing` tag is present in the theme’s `style.css` header and the Gutenberg plugin is not active to prevent this scenario.
These checks can be removed once Full Site Editing is completely merged into Core.
Props desrosj, marybaum, chanthaboune.
See #53410.
Built from https://develop.svn.wordpress.org/trunk@51193
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