Ask if server should start at next open port

In addition to #86, I think it would be great, if it would open the next open port as a default, like exactly the same what `create-react-app` does. So instead of prompting

```
Error: listen EADDRINUSE :::4000
at Object._errnoException (util.js:1024:11)
at _exceptionWithHostPort (util.js:1046:20)
at Server.setupListenHandle [as _listen2] (net.js:1351:14)
at listenInCluster (net.js:1392:12)
at Server.listen (net.js:1476:7)
...
```

... we should be able to decide if we want to open at `4001` instead.

Start by locating Backpack’s server startup and the handling of the EADDRINUSE error, then compare the requested prompt behavior with create-react-app. Done means startup offers the choice to use the next open port, such as 4001, instead of only failing on port 4000.

Written by the indexing model from the issue text.