misc: enable source maps for bundled runtime (#56289)

This PR enables the generation of source maps for the bundled runtimes. This is fast enough that we can just use the optimal source map option directly. This is useful if you're having some errors in Next.js itself.

Author: feedthejim

packages/next/webpack.config.js

@@ -134,6 +134,7 @@ module.exports = ({ dev, turbo, bundleType, experimental }) => {
       }.runtime.${dev ? 'dev' : 'prod'}.js`,
       libraryTarget: 'commonjs2',
     },
+    devtool: 'source-map',
     optimization: {
       moduleIds: 'named',
       minimize: true,