This pull request adds a registry for level screen providers in the fabric-screen-api-v1 module, which control the screens available from clicking 'Customize' on the create world screen. It can be used in the following way:

Identifier id = Identifier.of("example", "fabricland");
RegistryKey<WorldPreset> key = RegistryKey.of(RegistryKeys.WORLD_PRESET, key);

LevelScreenProviderRegistry.register(FABRICLAND_WORLD_PRESET, (parent, generatorOptionsHolder) -> {
	ChunkGenerator chunkGenerator = generatorOptionsHolder.selectedDimensions().getChunkGenerator();
	// Return a screen here that calls `parent.getWorldCreator().applyModifier(...)` to apply changes
});

To test this registry, a Fabricland chunk generator and world preset has been added to the fabric-screen-api-v1 test mod. The create world screen allows opening a customization screen which allows the background and outline blocks that are generated to be changed:

The corresponding names in Yarn mappings may be improved, so the LevelScreenProviderRegistry class name in this API should not be considered final.

Note that the LevelScreenProviderMixin class modifies Map.of rather than replacing the field value as other registry API mixins do; the Map.of method is more brittle, but seemingly is required as LevelScreenProvider is an interface. See this Discord message for more information.

Fixes #3926