Fix regen on modern versions

[SirYwell]

Overview

Fixes #2818 Fixes #2861

Description

We can greatly simplify the regeneration logic by relying on the server's chunk system instead of trying to run the generation ourselves. This works well on Paper, and probably good enough on Spigot. Conceptually, the change is similar to how a //lazycopy would work, just with the temporary world as a source. I think this is far better in terms of maintainability.

Tested on Paper 1.21(.1), only, but the code should work on older versions too and as there isn't much low-level implementation details, I also expect Spigot to work.

### Submitter Checklist
- [x] Make sure you are opening from a topic branch (**/feature/fix/docs/ branch** (right side)) and not your main branch.
- [x] Ensure that the pull request title represents the desired changelog entry.
- [x] New public fields and methods are annotated with `@since TODO`.
- [x] I read and followed the [contribution guidelines](https://github.com/IntellectualSites/.github/blob/main/CONTRIBUTING.md).

[Kadeluxe]

Works well and much faster than previous implementation, good job and thank you

[illyrius666]

can we get this merged asap? kinda need it :)