Add source maps to modern browsers dist

webpack.config.modern.js

@@ -32,6 +32,7 @@ module.exports = {
   mode: "production",
   entry: './src/lib/index.modern.ts',
   target: 'es5',
+  devtool: 'source-map',
   output: {
     filename: 'index.modern.js',
     path: path.resolve(__dirname, 'build'),