BlockState syntax for custom superflat world layer presets.

Currently when creating a superflat world, the syntax is:

minecraft:bedrock,2*minecraft:deepslate,minecraft:grass_block;minecraft:plains

, and the FlatLayerInfo only stores a block, always using the default block state.

You should be able to specify a BlockState like:

minecraft:bedrock, 2*minecraft:deepslate[axis=z], minecraft:grass_block[snowy=true]; minecraft:plains

, to make 2 layers of sideways deepslate with snowy grass_block on top.