Stop mangling server runtime in dev (#68641)

The server runtimes are currently all bundled in production mode with
full optimizaitons. This can make certain things hard in dev mode
because while most of your app will be unmangled anything that runs from
the server runtime will be runnign without useful debug information like
original function names. This PR updates the runtime configs to no
longer mangle the output.

packages/next/webpack.config.js

@@ -173,6 +173,7 @@ module.exports = ({ dev, turbo, bundleType, experimental }) => {
             format: {
               preamble: '',
             },
+            mangle: dev ? false : true,
           },
         }),
       ],