Duplicate NBT Flattener

Fixes corruption issues related to a severe bug found in interactions between Terrablender and outdated libraries frequently used in mods such as Vanilla Backport. These interactions can cause recursively duplicated surface rules in level.dat preventing the game from loading the world properly. Duplicate NBT Flattener cleans up this corrupted data in level.dat.

This mod does not need Terrablender or Vanilla Backport as dependencies, they are cited as example cases.