Revival of Add minetest.generate_biomes and minetest.generate_biome_dust

[kromka-chleba]

Attempted revival of #14260
Fixes #8329
Keep in mind that I have no C++ experience so this may be funny and could take some time.

Description of the PR:

As in the original: #14260

To do

This PR is READY

• Rebase (surprisingly resolving the conflict was trivial)
• Test and see that it still works
• Test it with the mapgen env
• Address reviewers' concerns from the original PR

I'm not sure about this #14260 (comment) but otherwise I hopefully addressed all review things.

EDIT: I think I fixed it here 05e4b7a

How to test

1. Clone this mod to your minetest/mods/
2. Enter the directory, switch to the use_core_biomegen branch
3. Make a new MTG world
4. Enable the lvm_example mod
5. Witness it working
6. Optionally switch to the master branch of lvm_example and see that it doesn't place biomes and dust.

Testing it in the mapgen env:

1. Create a flatgen directory in your minetest/mods/
2. Download the flatgen fork and put the files into flatgen
3. Make a new MTG world
4. Enable the flatgen mod
5. Witness it working

[kromka-chleba]

Regarding this #14260 (comment)

Do you want me to load a bigger VM and read y -1? With the mapgen env the VM should already be big enough thanks to the overgeneration margin. What should I do with arbitrarily-sized VMs made by modders?

EDIT: I think I fixed it here 05e4b7a

I also adjusted the examples (removed the manual offset). If that's not the solution then I'm waiting for reviewers to show me the proper fix.

[kromka-chleba]

@sfan5 would you be willing to continue reviewing this given that you were a reviewer for the original PR?
I hopefully addressed all your comments.