Add an interface to edit the .env file

I would advise against this. Just imagine you're using this to store the database credentials, and somebody manages to do a screw up on production wiping the entire file. Meaning if this was done through the UI, Drupal would crash and you have to scramble to find someone to fix the file on the server. You might also not want everyone to be able to get access to those secrets, having the possibility as a site admin to just enable this via a module, seems a bit as a risk to me.

I guess this can also happen there, but at least a sysadmin shouldn't make such a mistake :-).

Thanks for your input! You're right, probably not the best idea. It was worth considering though :).