New cave biome: Fiery Caverns

The fragmented Nether portals from 1.16 can now spawn deep underground, below y=0. The surrounding caves have become corrupted with lava, magma blocks and even netherrack. Zombified Piglins spawn down here and the Warden is unusually aggressive around these caves. Perhaps the legendary Wildfire (previously Hovering Inferno / Mob D) could spawn here, a new threat found deep below ground, alongside the Warden.