Add SpawnDimension to World Level.dat (Java)

Currently you can set the spawn dimension individually for players in Player.dat [or the player field in Level.dat] but the only way to to override the normal world spawn which is tied to the overworld, is to set. the "SpawnForced" flag. Unfortunately setting this flag also overrides beds, and respawn anchors. With the Nether update, and the addition of [experemental) support for custom. dimensions, custom map makers are wanting to start in the Nether or in their own custom dimension. It would be fantastic if Level.dat allowed for setting of SpawnDimension for the world settings.