Associate dotenv language with .env files

[remcohaszing]

Instead of associating a list of file extensions starting with .env with the dotenv language, The dotenv language is now associated with the .env* filename pattern. This also removes the need to write this file association to the user configuration.

The .flaskenv file is now associated via the .flaskenv filename instead of a file extension. Additionally the user-dirs.dirs file is associated with the dotenv language. This is an env file used to configure the XDG user directories.

File extensions I’m not familiar with, are left untouched.

Closes #107