Add support to Nukkit 2.0's LevelDB format

[joserobjr]

Is your feature request related to a problem? Please describe.
Nukkit 2.0 now targets the LevelDB format and Anvil is getting deprecated.

Describe the solution you'd like
Have an option to convert directly to a LevelDB format which Nukkit 2.0 can handle, so no more conversions are needed.

Describe alternatives you've considered
Create a new repository like the Region Manipulator which does the same thing but with LevelDB using pure Java if possible.

[joserobjr]

Nukkit seems to be using this LevelDB implementation: https://github.com/dain/leveldb
I think it would be a good idea to create a new base project without implementation, then make the Region Manipulator implement it and then make a new project which uses that LevelDB API to implement the base project, then Java2Nukkit World Converter would have an option settings which allows to choose between Anvil or LevelDB format.

[joserobjr]

An other idea is to create a new project which implements the new base project using the Region Manipulator

[suerion]

Would you add Pocketmin Support? yet, the converter work also for it?