move DefinePlugin to new webpack config location

webpack/client.config.js

@@ -29,7 +29,11 @@ module.exports = {
 	},
 	mode,
 	plugins: [
-		isDev && new webpack.HotModuleReplacementPlugin()
+		isDev && new webpack.HotModuleReplacementPlugin(),
+		new webpack.DefinePlugin({
+			'process.browser': true,
+			'process.env.NODE_ENV': JSON.stringify(mode)
+		}),
 	].filter(Boolean),
 	devtool: isDev && 'inline-source-map'
 };