Structures are no longer linked to Biomes via an instance field in the Biome class. Instead they are linked by registry keys. This means that standard biome spawning rules also apply to superflat worlds, instead of the hardcoded list of structures linked with superflat worlds by default. Changes the FabricStructureBuilder to allow adding structures to this default list.
* Add fix for handling removal of old structures.
This fix is required or else worlds where a mod is removed WILL NOT SAVE.
* Only mark for saving if the chunk has a null key
* Add label for issue being fixed
* Actually fix the redirect lol
* Licenses...
* Use ThreadLocal boolean instead of nullable unit
* Checkstyle is picky
* remove unnecessary `containsKey`
