A way for world generators to not be forced to run in an async task

[dktapps]

Problem description

Some generators, like Flat, do not require an async task, and using an async task to just copy a chunk is a waste of resources.

There may be other cases where it might be desirable to run parts of, or all, of generation on the main thread, such as when creating entities or tiles during a generation step (e.g. paintings or chests).

Proposed solution

Make Generator->generateChunk() and Generator->populateChunk() return Promise, so that individual generators can decide how they want to do async generation. This could be a promise resolved immediately on the main thread with a chunk copy, a PopulationTask, or perhaps some other mechanism entirely.

It may be more complex than this (e.g. a chunk source may be needed for adjacent chunks), but that's the basic premise.

Alternative solutions that don't require API changes