Let IChunkProvider allow or disallow mod generation

Added functionality to allow an IChunkProvider to determine whether mods should be able to generate there using the IWorldGenerator system.

Mods that add new dimensions will be able to take advantage of this functionality to effectively block other mods from populating their structures, trees, ores, etc. in that dimension.

The current implementation can be a little troublesome, because not all dimension modders (myself included) want other mods' features in their new dimension. Some dimensions are obviously more suited to this, but having the option to block it is incredibly helpful.