ItemBlock: reference blocks directly (take 2)

This was first attempted in f64dc01bd1, but reverted, since I hadn't considered how to handle stripping state data from blocks.

This removes the abusable API RuntimeBlockStateRegistry::fromTypeId() and related methods. These were only used to allow ItemBlocks to magically start referencing other blocks if the blocks were overridden by a plugin, but this was never a well-supported use-case anyway.

Instead of relying on RuntimeBlockStateRegistry, we remember the state that the block had during its constructor, and use that to normalize the non-item properties for asItem().

closes #5609

src/block/Light.php

@@ -34,7 +34,7 @@ final class Light extends Flowable{
 
 	private int $level = self::MAX_LIGHT_LEVEL;
 
-	protected function describeType(RuntimeDataDescriber $w) : void{
+	public function describeType(RuntimeDataDescriber $w) : void{
 		$w->boundedInt(4, self::MIN_LIGHT_LEVEL, self::MAX_LIGHT_LEVEL, $this->level);
 	}