Added StructureGrowEvent (#4354)

This event is currently fired for tree and bamboo growth. Its intended use is for any plant growth that affects multiple blocks at once.

TODO: We could explore using this for cacti and sugarcane?

src/event/block/StructureGrowEvent.php

@@ -0,0 +1,29 @@
+<?php
+
+declare(strict_types=1);
+
+namespace pocketmine\event\block;
+
+use pocketmine\block\Block;
+use pocketmine\event\Cancellable;
+use pocketmine\event\CancellableTrait;
+use pocketmine\world\BlockTransaction;
+
+/**
+ * Called when structures such as Saplings or Bamboo grow.
+ * These types of plants tend to change multiple blocks at once upon growing.
+ */
+class StructureGrowEvent extends BlockEvent implements Cancellable{
+	use CancellableTrait;
+
+	private BlockTransaction $transaction;
+
+	public function __construct(Block $block, BlockTransaction $transaction){
+		parent::__construct($block);
+		$this->transaction = $transaction;
+	}
+
+	public function getTransaction() : BlockTransaction{
+		return $this->transaction;
+	}
+}