use webpack for all builds

packages/netlify-cms-core/webpack.config.js

@@ -0,0 +1,27 @@
+const path = require('path');
+const FriendlyErrorsWebpackPlugin = require('friendly-errors-webpack-plugin');
+const baseConfig = require('../../webpack.config.js');
+
+module.exports = {
+  ...baseConfig,
+  context: path.join(__dirname, 'src'),
+  entry: './index.js',
+  module: {
+    noParse: /\.css$/,
+    ...baseConfig.module,
+  },
+  devServer: {
+    contentBase: './example',
+    watchContentBase: true,
+    quiet: true,
+    host: 'localhost',
+    port: 8080,
+  },
+  plugins: [
+    new FriendlyErrorsWebpackPlugin({
+      compilationSuccessInfo: {
+        messages: ['Netlify CMS is now running at http://localhost:8080'],
+      },
+    }),
+  ],
+};