Add target:browserslist to webpack config to specify what ES6 feature…

…s can be in runtime

Tells webpack whether it can use arrow functions or `let`/`const` in the generated runtime code.
Will start really work only after the fix in webpack/webpack#11754 is
released. Until then, arrow functions are incorrectly emitted even when they shouldn't.

packages/scripts/config/webpack.config.js

@@ -48,6 +48,7 @@ const cssLoaders = [
 
 const config = {
 	mode,
+	target: 'browserslist',
 	entry: {
 		index: path.resolve( process.cwd(), 'src', 'index.js' ),
 	},

webpack.config.js

@@ -64,6 +64,7 @@ module.exports = {
 		],
 	},
 	mode,
+	target: 'browserslist',
 	entry: gutenbergPackages.reduce( ( memo, packageName ) => {
 		const name = camelCaseDash( packageName );
 		memo[ name ] = `./packages/${ packageName }`;