Scrap TextContainer, make TranslationContainer immutable

TextContainer provided zero real value as a base of TranslationContainer, given that it required its own logic to be handled wherever accepted. As such, it's no better than a simple string.
Removing it also allows fixing an ambiguity when embedding translations inside other translations, allowing it to be made immutable.

src/command/CommandSender.php

@@ -23,14 +23,14 @@ declare(strict_types=1);
 
 namespace pocketmine\command;
 
-use pocketmine\lang\TextContainer;
+use pocketmine\lang\TranslationContainer;
 use pocketmine\permission\Permissible;
 use pocketmine\Server;
 
 interface CommandSender extends Permissible{
 
 	/**
-	 * @param TextContainer|string $message
+	 * @param TranslationContainer|string $message
 	 */
 	public function sendMessage($message) : void;