Investigate integration with FSE to implement offline/500 error templates

[westonruter]

Instead of adding a 404.php template file to a theme, Full Site Editing promises the ability to edit the 404 template in the Site Editor and customize it using blocks however the site owner sees fit. This same capability should be provided for customizing the Offline and 500 error templates, rather than requiring a theme author to add offline.php and 500.php respectively.