User server.js for single executables

The [default value](https://docs.npmjs.com/misc/scripts#default-values) for the `start` script is to exec `node server.js`, so if there's no specific `start` script on package.json and the package has only a single executable (like a CLI), that executable file should be named `server.js` so it can be launched with `npm` in a standard and homogeneous way.
