WorldProviderManager: added getAvailableProviders()

this will be used by standalone CLI conversion tool

src/world/format/io/WorldProviderManager.php

@@ -102,6 +102,13 @@ abstract class WorldProviderManager{
 		return $result;
 	}
 
+	/**
+	 * @return string[]|WorldProvider[]
+	 */
+	public static function getAvailableProviders() : array{
+		return self::$providers;
+	}
+
 	/**
 	 * Returns a WorldProvider by name, or null if not found
 	 *