Add a ddev rector command

[JPustkuchen]

Just came across rector once again and I think it might be helpful as ddev command!

I'm not sure if we should also include https://github.com/palantirnet/drupal-rector or at least take a look at the latest state. The "drupal 9" description seems outdated or is the project outdated in general?

https://github.com/nkoporec/social-ddev already did the inclusion into ddev as it seems? So perhaps this is a quick-win?