Allow Haxescripts to be used on custom resources

When creating [custom resources](https://docs.godotengine.org/en/stable/getting_started/step_by_step/resources.html#creating-your-own-resources) it would be awesome to have the ability to use Haxe Scripts in the Script property of a resource. 
An example could be a castledb integration that embeds and exposes the DB for other nodes to use.