New mob: Deer (with 3 variants)

A new mob will appear in the forests: the deer. It will have three variants:
- A “cold” variant, dark brown in color.
- A “template” variant, light brown in color.
- A “warm” variant, very dark grayish in color.

When you kill a deer, it will drop venison, which will be the same as the other meats in the game. It may also drop deer leather/deer hide. This deer will appear in most of the game’s forests (with the exception of fantasy biomes such as the Pale Garden).