Update theme.json classes for WordPress 5.9.

This commit ports to core the changes to the classes that deal with theme.json code.

See #54336.
Props oandregal, spacedmonkey, noisysocks, hellofromtonya, youknowriad.

git-svn-id: https://develop.svn.wordpress.org/trunk@52049 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/theme/themeDir.php

@@ -162,6 +162,7 @@ class Tests_Theme_ThemeDir extends WP_UnitTestCase {
 			'camelCase',
 			'REST Theme',
 			'Block Theme',
+			'Block Theme Child Theme',
 		);
 
 		sort( $theme_names );