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.

git-svn-id: https://develop.svn.wordpress.org/trunk@51657 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/edit-form-blocks.php

@@ -31,7 +31,7 @@ $current_screen->is_block_editor( true );
 // Default to is-fullscreen-mode to avoid jumps in the UI.
 add_filter(
 	'admin_body_class',
-	function( $classes ) {
+	static function( $classes ) {
 		return "$classes is-fullscreen-mode";
 	}
 );