Add FrEee Fusion Mod

**What do you want to see?**
Add the FrEee Fusion Mod, a port of BlackKnyght and Bearclaw's Fusion Mod to FrEee.

**How would it help players?**
Another mod to play 🙂

**Alternatives**
Put it in a separate repo?

**Screenshots and mockups**
N/A

**Things to consider**
What if Bearclaw wants to update the mod? Does he know how to use git?

Should the mod be in its own repo instead of this repo? That would make it easier to update out of sync with the game itself, but then mod updates would have to be manually synced to supporting game updates.

**Related issues**
N/A