Add beforeunload string for shiny updates.

See #31769.
Props johnbillion, adamsilverstein, jorbin. 



git-svn-id: https://develop.svn.wordpress.org/trunk@31989 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/script-loader.php

@@ -537,6 +537,7 @@ function wp_default_scripts( &$scripts ) {
 				'updatingMsg'       => __( 'Updating... please wait.' ),
 				'updatedMsg'        => __( 'Update completed successfully.' ),
 				'updateCancel'      => __( 'Update canceled.' ),
+				'beforeunload'      => __( 'Plugin updates may not complete if you navigate away from this page.' ),
 			)
 		) );