Stem: implement facing property

fixes #5858

technically speaking, the sideways states for non-fully-grown stems shouldn't exist, but they do in Bedrock, and changing this code to split non-fully-grown stems from fully grown ones would likely require BC breaks.
This was the minimum necessary to achieve the desired functionality.

src/data/bedrock/block/convert/BlockStateDeserializerHelper.php

@@ -236,9 +236,12 @@ final class BlockStateDeserializerHelper{
 
 	/** @throws BlockStateDeserializeException */
 	public static function decodeStem(Stem $block, BlockStateReader $in) : Stem{
-		//TODO: our stems don't support facings yet (facing_direction)
-		$in->todo(BlockStateNames::FACING_DIRECTION);
-		return self::decodeCrops($block, $in);
+		//In PM, we use Facing::UP to indicate that the stem is not attached to a pumpkin/melon, since this makes the
+		//most intuitive sense (the stem is pointing at the sky). However, Bedrock uses the DOWN state for this, which
+		//is absurd, and I refuse to make our API similarly absurd.
+		$facing = $in->readFacingWithoutUp();
+		return self::decodeCrops($block, $in)
+			->setFacing($facing === Facing::DOWN ? Facing::UP : $facing);
 	}
 
 	/** @throws BlockStateDeserializeException */