revert: revert node-externals disabling in dev mode

[clarkdo]

This pr is reverting #5414 since including all node_modules will cause problems.

So for fixing dev mode memory leak issue, we suggest user using build.standalone: true.

standalone mode will include all node_modules into server bundle file and avoid shared Vue module in Node.js main process which leads to memory leak in dev mode.

But one thing need to be notified is: If all node_modules all bundled into server.js by webpack, the modules which are using Node.js globals and modules like __dirname won't work as be resolved successfully by webpack, so those modules need to be declared in webpack externals.

For example: firebase and its dependency grpc are using __direname, so the config would be like:

// nuxt.config.js
export default {
  build: {
    standalone: true,
    extend(config, { isDev, isServer }) {
      if (isDev && isServer) {
        config.externals = [
          'grpc',
          '@firebase/app',
          '@firebase/auth',
          '@firebase/firestore',
          '@firebase/storage'
        ]
      }
    }
  }
}