Added ClosureCommand (#6063)

this is intended to replace PluginCommand and CommandExecutor, both of which are overengineered and unfit for purpose.
Allowing a closure allows much greater flexibility.

We can't use this within the core yet, as plugins will expect PluginBase->getCommand() to return PluginCommand (with its associated setExecutor() and similar APIs).
However, I think this is useful enough to add by itself.

src/command/ClosureCommand.php

@@ -0,0 +1,60 @@
+<?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\command;
+
+use pocketmine\lang\Translatable;
+use pocketmine\utils\Utils;
+
+/**
+ * @phpstan-type Execute \Closure(CommandSender $sender, Command $command, string $commandLabel, list<string> $args) : mixed
+ */
+final class ClosureCommand extends Command{
+	/** @phpstan-var Execute */
+	private \Closure $execute;
+
+	/**
+	 * @param string[] $permissions
+	 * @phpstan-param Execute $execute
+	 */
+	public function __construct(
+		string $name,
+		\Closure $execute,
+		array $permissions,
+		Translatable|string $description = "",
+		Translatable|string|null $usageMessage = null,
+		array $aliases = []
+	){
+		Utils::validateCallableSignature(
+			fn(CommandSender $sender, Command $command, string $commandLabel, array $args) : mixed => 1,
+			$execute,
+		);
+		$this->execute = $execute;
+		parent::__construct($name, $description, $usageMessage, $aliases);
+		$this->setPermissions($permissions);
+	}
+
+	public function execute(CommandSender $sender, string $commandLabel, array $args){
+		return ($this->execute)($sender, $this, $commandLabel, $args);
+	}
+}