Adds a magma oceans region to the nether outside the existing nether caverns, which can be reached via tunnels.
Other misc changes:
* chatcomment nether_whereami, a debug aid for knowing which perlin-noise region you are in
* Nether ores no longer obtainable on the ceiling
* Move crafts into crafts.lua
* Add steam to lava cooling, and play bubbling lava upon death by lava
* Add cracked netherbrick
a decorative node which can only be obtained from dungeons or structures
I encourage someone to improve or replace the cracked netherbrick texture. For copyright purposes it's currently a derivative work (by me, 2020) from nether_brick.png, which is licensed under CC BY-SA 3.0 by PilzAdam, so it can fall under the "All other media" PilzAdam's credit in readme.md rather than need its own entry.
The biomes-based mapgen was creating the same amount of lava as the original nether mapgen, but it doesn't have the same issue of chunk emerge order sometimes causing lava in the overdraw regions to get removed. This adjustment will hopefully balance that a little.
Also makes glowstone stalactite a bit rarer.
Adjusting lava ore scarcity in the biomes-mapgen doesn't cause forwards or backwards compatibilty issues with existing maps, likewise with schematic rarety like glowstone stalactites, so can afford to fiddle and tune.
Prevents the c++/native mapgen from placing ores or decorations (when useBiomes is true).
The biomes-based implementation is faster, more deterministic, and keeps its dungeons, but requires MT 5.1 features to work.
Also
* Glowstone & Netherrack stalactites
* Include the new decorations in mapgen_nobiomes
* Decorate dungeons - Add windows and glowstone "chandeliers" to some rooms
* Configurable Nether floor and ceiling depths
