Apply has-custom-width class on frontend

Fixes a bug where the inner button was not being set correctly to 100%
width on the frontend, due to the missing classname.`

Author: Staci Cooper

packages/block-library/src/button/save.js

@@ -41,7 +41,7 @@ export default function save( { attributes, className } ) {
 	// A title will no longer be assigned for new or updated button block links.
 
 	const wrapperClasses = classnames( className, {
-		[ `wp-block-button__width-${ width }` ]: width,
+		[ `has-custom-width wp-block-button__width-${ width }` ]: width,
 	} );
 
 	return (