Extract GeneratorExecutor system from World, v2 (#6682)

- `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()`

tests/phpstan/configs/actual-problems.neon

@@ -1272,18 +1272,18 @@ parameters:
 			count: 1
 			path: ../../../src/world/format/io/region/RegionLoader.php
 
-		-
-			message: '#^Dynamic new is not allowed\.$#'
-			identifier: pocketmine.new.dynamic
-			count: 1
-			path: ../../../src/world/generator/GeneratorRegisterTask.php
-
 		-
 			message: '#^Method pocketmine\\world\\generator\\biome\\BiomeSelector\:\:pickBiome\(\) should return pocketmine\\world\\biome\\Biome but returns pocketmine\\world\\biome\\Biome\|null\.$#'
 			identifier: return.type
 			count: 1
 			path: ../../../src/world/generator/biome/BiomeSelector.php
 
+		-
+			message: '#^Dynamic new is not allowed\.$#'
+			identifier: pocketmine.new.dynamic
+			count: 1
+			path: ../../../src/world/generator/executor/GeneratorExecutorSetupParameters.php
+
 		-
 			message: '#^Cannot call method getBiomeId\(\) on pocketmine\\world\\format\\Chunk\|null\.$#'
 			identifier: method.nonObject