fix(config): default Scribunto engine param to null

Otherwise, extensions that depend on Scribunto will overwrite the param when calling ->Scribunto().

Author: SomeMWDev

config/MWCExtensions.php

@@ -755,7 +755,7 @@ public function Screenplay(): self {
 	public const SCRIBUNTO_ENGINE_LUASANDBOX = 2;
 
 	public function Scribunto(
-		int $engine = self::SCRIBUNTO_ENGINE_LUASTANDALONE,
+		?int $engine = null,
 		array $engineConfOverrides = []
 	): self {
 		switch ( $engine ) {