Add a hook to enable blocks to react to projectiles colliding with them

this enables implementing blocks such as the target block.

src/block/Block.php

@@ -32,6 +32,7 @@ use pocketmine\block\utils\SupportType;
 use pocketmine\data\runtime\RuntimeDataReader;
 use pocketmine\data\runtime\RuntimeDataWriter;
 use pocketmine\entity\Entity;
+use pocketmine\entity\projectile\Projectile;
 use pocketmine\item\enchantment\VanillaEnchantments;
 use pocketmine\item\Item;
 use pocketmine\item\ItemBlock;
@@ -626,6 +627,13 @@ class Block{
 		return null;
 	}
 
+	/**
+	 * Called when a projectile collides with one of this block's collision boxes.
+	 */
+	public function onProjectileHit(Projectile $projectile, RayTraceResult $hitResult) : void{
+		//NOOP
+	}
+
 	/**
 	 * @return AxisAlignedBB[]
 	 */