Coding Standards: Use static closures when not using $this.

When a closure does not use `$this`, it can be made `static` for improved performance.

Static closures are supported in PHP since PHP 5.4. ​

Props jrf, hellofromTonya, swissspidy, SergeyBiryukov.
See #53359.
Built from https://develop.svn.wordpress.org/trunk@51657


git-svn-id: http://core.svn.wordpress.org/trunk@51263 1a063a9b-81f0-0310-95a4-ce76da25c4cd

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

@@ -636,7 +636,7 @@ class WP_Theme_JSON {
 
 		$declaration_block = array_reduce(
 			$declarations,
-			function ( $carry, $element ) {
+			static function ( $carry, $element ) {
 				return $carry .= $element['name'] . ': ' . $element['value'] . ';'; },
 			''
 		);