Make ShiftExistingBiomes available via API

[Treer]

Fixes bug with wrong new_y_min when shifting a biome below the nether floor, and makes the ShiftExistingBiomes function available to other mods via the nether global, since it's not a simple function and biomes would also need to be shifted if another mod wants to add a second nether layer.

I realised ShiftExistingBiomes() would need to be exposed while responding to ywwv's suggestion of adding more layers, however it's not clear whether anyone will ever care to do that, so it's hardly urgent, but also pretty minor.

'll merge this in eventually if there's no comments or change requests. I'll first implement improvements raised by SmallJoker (now done)

[SmallJoker]

Is there yet any API documentation? As the mod grows in features, it might make sense to list them somewhere.

[Treer]

API documentation added, and those other suggestions done.

[SmallJoker]

:+1: Looks good. Feel free to merge whenever you want. Also thank you for the effort to write the documentation for this API. This will surely ease writing of mods that add even more content to nether.

[Treer]

thanks for looking over it, I was about to merge because I'm keen to get contentDB updated with all the recent fixes, but decided to test this PR properly with an example mod, one thing lead to another and I've uncovered a few more minor bugs in the nether mod and also want to expose the nether decoration schematics in the api, so there'll be another commit coming.

[Treer]

A demo mod for extending the nether, really it's more an elaborate test that the extensibility works, but I'll post it on the forums once a new version of the Nether is out.