Custom world generation/position source

• •  type: The namespaced id of the position source type. Must be one of block and entity.

    If type is block, additional field is as follow:

  •  pos: The block position.
    • : X.
    • : Y.
    • : Z.

    If type is entity, additional fields are as follows:

  •  source_entity: The int-array UUID of an entity.
  •  y_offset: (optional, defaults to 0.0) The Y offset from entity's feet.