Editor: Improve the append_to_selector method.

Improve `append_to_selector` method in `WP_Theme_JSON` by checking to see if string contains a common before imploding / exploding string, which improves performance. 

Originally developed and tested in [WordPress/gutenberg#47833 Gutenberg PR 47833].

Props spacedmonkey, flixos90, mukesh27, joemcgill, wildworks, oandregal, mamaduka.
Fixes #58231.

git-svn-id: https://develop.svn.wordpress.org/trunk@55907 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-theme-json.php

@@ -789,6 +789,9 @@ class WP_Theme_JSON {
 	 * @return string The new selector.
 	 */
 	protected static function append_to_selector( $selector, $to_append, $position = 'right' ) {
+		if ( ! str_contains( $selector, ',' ) ) {
+			return 'right' === $position ? $selector . $to_append : $to_append . $selector;
+		}
 		$new_selectors = array();
 		$selectors     = explode( ',', $selector );
 		foreach ( $selectors as $sel ) {