expanded support for structure variants
added end city pieces generation (WIP)
added outer end voids (MC-159283)
fixed several issues with the availablity of biomes for layered generation
fixed that about half of 1.18/1.19 end gateways have wrong positions
Different versions, dimensions and variations are now bundled into a single Generator structure.
Areas (x,z,w,h) are largely replaced with a Range structure that supports 3D cuboids with scaling.
The scale now acts as a generalization that replaces layer access. Direct layer access will
have to be version specific, using the LayerStack.
(I.e. this update will unfortunately break much of the API.)
