Enable server sourcemaps (#2242)

* add sourcemaps for server code

* leave dev config alone

next.config.js

@@ -8,7 +8,12 @@ module.exports = {
   reactStrictMode: true,
   i18n,
   output: 'standalone',
-  webpack: (config, { webpack, isServer }) => {
+  webpack: (config, { dev, webpack, isServer }) => {
+    // Enable source maps in development mode and for server-side rendering
+    if (!dev && isServer) {
+      config.devtool = 'source-map';
+    }
+
     if (isServer) {
       // Module not found
       config.plugins.push(