Writes .env.example from Gamend.Settings.all/0, grouped and
commented from each setting's declaration.
mix gamend.settings.env_example # write .env.example
mix gamend.settings.env_example --check # fail if it is out of date
mix gamend.settings.env_example -o PATH # write somewhere elseThe file was hand-maintained until now, and drifted: 31 variables the code
reads were documented nowhere. Generating it means a setting is documented
the moment it is declared, and --check keeps it that way in CI.