Add getter methods to retrieve listener and inventory close listener

allows developers to retrieve the current listener, extend its behavior, and then re-set it.
This provides more flexibility in chaining or extending listener functionalities without losing the original behavior.

```php
$listener = $menu->getInventoryCloseListener();
$menu->setInventoryCloseListener(function(Player $player, Inventory $inventory) use($listener) : void{
	$listener($player, $inventory);
	// place additional inventory close checks here
});
```

Author: Muqsit

src/muqsit/invmenu/InvMenu.php

@@ -83,6 +83,13 @@ public function setName(?string $name) : self{
 		return $this;
 	}
 
+	/**
+	 * @return (Closure(InvMenuTransaction) : InvMenuTransactionResult)|null
+	 */
+	public function getListener() : ?Closure{
+		return $this->listener;
+	}
+
 	/**
 	 * @param (Closure(InvMenuTransaction) : InvMenuTransactionResult)|null $listener
 	 * @return self
@@ -92,6 +99,13 @@ public function setListener(?Closure $listener) : self{
 		return $this;
 	}
 
+	/**
+	 * @return (Closure(Player, Inventory) : void)|null
+	 */
+	public function getInventoryCloseListener() : ?Closure{
+		return $this->inventory_close_listener;
+	}
+
 	/**
 	 * @param (Closure(Player, Inventory) : void)|null $listener
 	 * @return self