Added Darkest Dungeon (GoG) derivation

[SrEstegosaurio]

Adds a Darkest Dungeon derivation. (Link to the game website).

This is related to the issue #110. While this is just a single game and not a builder I think it still has value because:

1. Immediately provides another game to possible end users.
2. It serves as an starting point and I would very much appreciate constructive criticism. I'm a bit noob with nix. 😅

WIP

• I still need to iron out some things like gamemode support, a proper icon and maybe clean up some bits here and there.
• Documentation is also missing since I feel that a disclaimer stating that this is not piracy and the end user need to provide the original installer.