Makes it so old maps on modern servers will have a min y = 0, meaning you can't build under y=0, so it behaves as they used to.

To make this happen a bunch of horrible hacks were employed:

• Had to copy-in CraftServer#createWorld to allow a dimension that isn't one of the defaults
• Had to register the custom dimension type via unspeakable means
  • Had to implement my own support for modifying dimension type, jumping thru alot of hoops, including the use of unsafe
• Had to register a data fixer so worlds upgrading past 1.17 get their dimension changed
• Turned pgm into a paper plugin, to have access to bootstrap-time code
  • This resulted in bytecode remapping no longer applying
    • Had to account InventoryView being an interface instead of a class now
    • Replaced all usages manually with a platform-dependant impl.

This would've been alot easier of paper supported modifying the dimensionType as part of world creator, and supported dimension type as a modifiable registry.

It can still be cleaned up now that i got it to a stage that works, but i don't think there's a work-around for the use of unsafe to access unmodifiable fields.

Ontop of this all, it seems that if the world is created with a different dimensions some world conversion doesn't run, leaving fences or glass panes unconnected. Opened PaperMC/DataConverter#22 to see if we can get any help for this, in the form of a fix, or a workaround of some sorts
By registering our own data fix that modifies the dimension we've been able to workaround this one