Add RegionPosition class, replace all usages of ChunkPosition as a region with it.

[NotStirred]

This should have zero impact on performance, but makes things much clearer and adds a nice layer of type safety on top.

I think the name of ChunkPosition#asRegionPosition is worth discussing. I didn't want to change the return type of the existing getRegionPosition in case any plugins use it as I'm unsure. Maybe none do?