- `AsyncGeneratorExecutor` class added that encapsulates the logic of generating chunks using async tasks as previously
- `GeneratorExecutor` interface added that can be implemented to provide chunks in other ways
- `SyncGeneratorExecutor` which invokes the generator directly on the main thread, useful for simple generators like `Flat` where async tasks are not needed
- Some redundant APIs were removed from `World` (these will probably come back as deprecated stubs for the remainder of 5.x, but I was having too much fun deleting code)
- Removed internal `World->registerGeneratorToWorker()` (no longer useful)
- `World` now invokes generator executor instead of posting AsyncTasks directly
- Some internal classes moved to `pocketmine\world\generator\executor` (PopulationTask excluded because plugins use it in lieu of being able to regenerate chunks
- Generators can opt into main-thread execution by setting the `$fast` parameter to `true` in `GeneratorManager::register()`
