Add dotenv support

[KilianVounckx]

As discussed on zulip, there should be some kind of support for reading environment variables via dotenv files. I think there are two possibilities here.

1. Have a .env file be automatically loaded on any program using the platform.
2. Have a function like Dotenv.load or Env.loadDotenv of type Task {} _ which loads the dotenv file and makes later calls to Env.var use the new environment variables.