Make it closer to a boilerplate
Maybe leave the server entry and client entry to user-land, by running `ream dev` in current directory, it will use `./index.server.js` and `index.client.js` as entry files when they exist, currently Ream uses the built-in ones: [client.js](https://github.com/ream/ream/blob/b3dfded417050dd046311e15990598b897986593/packages/ream/app/entries/client.js) [server.js](https://github.com/ream/ream/blob/b3dfded417050dd046311e15990598b897986593/packages/ream/app/entries/server.js)