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/event/entity/ItemMergeEvent.php

@@ -0,0 +1,52 @@
+<?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\event\entity;
+
+use pocketmine\entity\object\ItemEntity;
+use pocketmine\event\Cancellable;
+use pocketmine\event\CancellableTrait;
+
+/**
+ * Called when an item entity tries to merge into another item entity.
+ *
+ * @phpstan-extends EntityEvent<ItemEntity>
+ */
+class ItemMergeEvent extends EntityEvent implements Cancellable{
+	use CancellableTrait;
+
+	public function __construct(
+		ItemEntity $entity,
+		protected ItemEntity $target
+	){
+		$this->entity = $entity;
+	}
+
+	/**
+	 * Returns the merge destination.
+	 */
+	public function getTarget() : ItemEntity{
+		return $this->target;
+	}
+
+}