text: Title: Improve UX around changing displacy's default port
  Body: The default port of `displacy.serve` is 5000, which sometimes causes conflicts as described in https://github.com/explosion/spaCy/pull/11793.
  
  `displacy.serve` already has a port keyword arg, but perhaps adding a global setting/env var would be good, too. Additionally then, it would be nice to try and catch port conflicts and throw a custom spaCy error that tells users how to change the port.
  