Save the fish config to `$__fish_user_data_dir`

[kabouzeid]

This path makes more sense and doesn't clutter the user's dotfiles.

See also: https://fishshell.com/docs/current/language.html#configuration-files

[kabouzeid]

I'd consider it data because it is autogenerated and not meant to be edited by the user. Clean up is not strictly necessary because the new location will be sourced after the old location. But I agree, it'd be best to remove the old file.

[kabouzeid]

I've added clean-up code + a small sanity check as to not accidentally delete a user created file.

[jfly]

Thanks!