Related to #15

While for the nether and end checking the parent folder to find level.dat is sufficient, custom dimensions as added by datapacks in 1.16+ are a couple subfolders deeper, so the level.dat is not found automatically by bluemap.

World locations:
world/ - overworld
world/DIM1 - end
world/DIM-1 - nether
world/dimensions/{namespace}/{dimension_name} - custom datapack dimension

As a temporary solution, I can just symlink world/level.dat to world/dimensions/{namespace}/level.dat, but it would be nice if the mod could automatically look a couple folders higher.
Once the level.dat can be found, everything seems to work perfectly fine when the dimension is added to the config.