Update background loading resource_queue.gd. to 3.1, Starting with C++ necessary?

https://docs.godotengine.org/en/3.1/tutorials/io/background_loading.html
I'm background loading large images because Godot will freeze or skip frames if I don't.

I really wish the docs would not start with C++ code. It took a lot of perseverance to figure out I actually only needed a few lines from the example code at the bottom of the page and I would not have to wrap my mind around the first rather large indigestible junk of information.

The GDScript example code seems to work great! Thanks to whoever wrote this and provided the resource_queue.gd!

resource_queue.gd seems to work, but brings up a shitload of warnings.
One of them coming up every time you want to `queue.queue_resource()` saying `ResourceLoader.has()` is depreciated ([why is it still in the class docs then?](https://docs.godotengine.org/en/3.1/classes/class_resourceloader.html#class-resourceloader-method-has)), and I should use `has_cached` or `exists` instead. Not sure which is the correct one in this case (line 34 in resource_queue.gd), but `exists `seems not to cause any troubles.

Since it is part of the official docs, I think it would be great if resource_queue.gd would not give off any warnings and I could use it just as is without having to change depreciated code.