Add wp-polyfill dependency via webpack plugin

Enable `injectPolyfill` option in the webpack plugin instead of manually
injecting the dependency via PHP on script registration.

This will enable the polyfill for consumers of wp-scripts default
webpack config.

lib/client-assets.php

@@ -211,7 +211,7 @@ function gutenberg_register_packages_scripts() {
 		gutenberg_override_script(
 			$handle,
 			gutenberg_url( $gutenberg_path ),
-			array_merge( $dependencies, array( 'wp-polyfill' ) ),
+			$dependencies,
 			filemtime( $path ),
 			true
 		);

packages/scripts/config/webpack.config.js

@@ -67,7 +67,7 @@ const config = {
 		// WP_LIVE_RELOAD_PORT global variable changes port on which live reload works
 		// when running watch mode.
 		! isProduction && new LiveReloadPlugin( { port: process.env.WP_LIVE_RELOAD_PORT || 35729 } ),
-		new DependencyExtractionWebpackPlugin(),
+		new DependencyExtractionWebpackPlugin( { injectPolyfill: true } ),
 	].filter( Boolean ),
 	stats: {
 		children: false,