ItemBlock: drop the charade about overriding built-in block types

this allows cleaning up a whole lot of abusable mess from the API, and we never properly supported overriding built-in block types anyway.

src/block/Block.php

@@ -214,11 +214,11 @@ class Block{
 		return $writer->getValue();
 	}
 
-	protected function describeType(RuntimeDataDescriber $w) : void{
+	public function describeType(RuntimeDataDescriber $w) : void{
 		//NOOP
 	}
 
-	protected function describeState(RuntimeDataDescriber $w) : void{
+	public function describeState(RuntimeDataDescriber $w) : void{
 		//NOOP
 	}