Updates theme validation to accept block themes. This replaces the requirement for an `index.php` with a requirement for either an `index.php`, `/templates/index.html` or the deprecated `/block-templates/index.html`.
Validation is updated for theme uploads, within `WP_Theme::__construct` and `validate_current_theme()`.
A block theme using the deprecated file structure is now included in the unit tests.
Props peterwilsoncc, sergeybiryukov, hellofromtonya, costdev, azaozz, gziolo, FlorianBrinkmann, Boniu91, aristath, poena, audrasjb.
Fixes#55754.
git-svn-id: https://develop.svn.wordpress.org/trunk@53416 602fd350-edb4-49c9-b593-d223f7449a82
