GroundZero Resource Pack v45 - Zombie Nests

Added:
- Custom 3D Zombie Nest Core model (Heart of the Sea CMD 9600)
- assets/minecraft/textures/item/gz/zombie_nest_core.png
- assets/minecraft/models/item/gz/zombie_nest_core.json
- Zombie nest spawn scream: groundzero:zombienest.spawn

The vanilla Heart of the Sea model remains untouched unless CustomModelData=9600.
