Implement dropped item merging (#4419)

- The following classes have been added:
  - `ItemMergeEvent`
  - `ItemEntityStackSizeChangeAnimation`
- The following API methods have been added:
  - `ItemEntity->isMergeable()`
  - `ItemEntity->tryMergeInto()`
  - `ItemEntity->setStackSize()`

src/entity/animation/ItemEntityStackSizeChangeAnimation.php

@@ -0,0 +1,40 @@
+<?php
+
+/*
+ *
+ *  ____            _        _   __  __ _                  __  __ ____
+ * |  _ \ ___   ___| | _____| |_|  \/  (_)_ __   ___      |  \/  |  _ \
+ * | |_) / _ \ / __| |/ / _ \ __| |\/| | | '_ \ / _ \_____| |\/| | |_) |
+ * |  __/ (_) | (__|   <  __/ |_| |  | | | | | |  __/_____| |  | |  __/
+ * |_|   \___/ \___|_|\_\___|\__|_|  |_|_|_| |_|\___|     |_|  |_|_|
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * @author PocketMine Team
+ * @link http://www.pocketmine.net/
+ *
+ *
+*/
+
+declare(strict_types=1);
+
+namespace pocketmine\entity\animation;
+
+use pocketmine\entity\object\ItemEntity;
+use pocketmine\network\mcpe\protocol\ActorEventPacket;
+use pocketmine\network\mcpe\protocol\types\ActorEvent;
+
+final class ItemEntityStackSizeChangeAnimation implements Animation{
+
+	public function __construct(
+		private ItemEntity $itemEntity,
+		private int $newStackSize
+	){}
+
+	public function encode() : array{
+		return [ActorEventPacket::create($this->itemEntity->getId(), ActorEvent::ITEM_ENTITY_MERGE, $this->newStackSize)];
+	}
+}