Make msgbox.js a module

Refs: #8081

binaries/data/mods/mod/gui/msgbox/msgbox.js

@@ -2,7 +2,7 @@
  * Currently limited to at most 3 buttons per message box.
  * The convention is to have "cancel" appear first.
  */
-function init(data)
+export function init(data)
 {
 	// Set title
 	Engine.GetGUIObjectByName("mbTitleBar").caption = data.title;

binaries/data/mods/mod/gui/msgbox/msgbox.xml

@@ -3,7 +3,7 @@
 <objects>
 
 	<script directory="gui/common/"/>
-	<script directory="gui/msgbox/"/>
+	<script module="gui/msgbox/msgbox.js"/>
 
 	<!-- Fade out the background because it's non-interactable -->
 	<object sprite="ModernFade" type="image"/>