Use ES6 and PHP 7.4 syntax

Co-authored-by: Weston Ruter <westonruter@gmail.com>

Author: manzoorwanijk

src/wp-includes/class-wp-script-modules.php

@@ -394,12 +394,12 @@ public function print_script_module_translations(): void {
 			}
 
 			$output = <<<JS
-( function( domain, translations ) {
-	var localeData = translations.locale_data[ domain ] || translations.locale_data.messages;
+( ( domain, translations ) => {
+	const localeData = translations.locale_data[ domain ] || translations.locale_data.messages;
 	localeData[""].domain = domain;
 	wp.i18n.setLocaleData( localeData, domain );
 } )( "{$domain}", {$json_translations} );
-JS;
+			JS;
 
 			$source_url = rawurlencode( "{$id}-js-module-translations" );
 			$output    .= "\n//# sourceURL={$source_url}";