Add Site Editor and PHP changes from Gutenberg 10.1 - 11.9

- First pass at adding the site editor from the Gutenberg plugin to
  wp-admin/site-editor.php.
- Adds miscellaneous PHP changes from Gutenberg 10.1 - 11.9.

Follows [52042].
See #54337.
Props youknowriad, aristath, hellofromtonya, gziolo.


git-svn-id: https://develop.svn.wordpress.org/trunk@52069 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/theme.php

@@ -4075,3 +4075,14 @@ function create_initial_theme_features() {
 		)
 	);
 }
+
+/**
+ * Returns whether the current theme is a block-based theme or not.
+ *
+ * @since 5.9.0
+ *
+ * @return boolean Whether the current theme is a block-based theme or not.
+ */
+function wp_is_block_template_theme() {
+	return is_readable( get_theme_file_path( '/block-templates/index.html' ) );
+}