Add tweaks to disable terrain/structure generation

[nzbasic]

Adds 3 new tweaks:

• disableChunkTerrainGeneration [default false]: when true sets all blocks to air upon new chunk generation, keeping biome data
• disableWorldTypeChunkPopulation [default false]: when true redirects calls to the dimension's IChunkProvider.populate (preventing e.g. flowers, ores, vanilla overworld structures)
• disableModdedChunkPopulation [default false]: when true redirects calls to forge's GameRegistry.generateWorld (preventing e.g. Natura's cloud gen, Draconic Chaos Island gen)

When all three are enabled you get true void dims for all vanilla/modded dimensions with biomes preserved

The main reason for this addition is to give improved options for void based GTNH playthroughs. Currently creating a Garden of Grind world involves removing the default world gen mod and creating a superflat world with a void preset which is fixed to 1 biome. Now with these configs we can enjoy the normal GTNH RWG biomes in the overworld and start allowing exploration of other dimensions in a true to skyblock fashion.

Dim screenshots:

Nether (all configs enabled)

TF (all configs enabled)

Overworld (RWG) (all configs enabled)

TF (only disableChunkTerrainGeneration = true)

Overworld (only disableChunkTerrainGeneration = true)

[LazyFleshWasTaken]

Can also be used to troubleshoot structure generation if needed.