Expose get_autoload_list to gdscript

[vaartis]

Expose get_autoload_list as a typed array to gdscript, since gdscript does not have an ordered array type. It's useful in GDScript too (I'd just tried to find something like this for my own project and saw it wasn't available in GDScript), so it should probably be available.

[Mickeon]

Although I'm not vehemently against this, could you provide the use case you need this for, and preferably open a Proposal to gather user interest? It is kinda possible to do this already, by opening the raw project.godot as a ConfigFile, but I can understand it's not exactly accessible, nor obvious.

[vaartis]

My specific use case for this is as follows: I have a function that loads saved data for some scene. Together with that I need to also load data into some auto-loaded nodes, of which there are several. I'd like it if I could have a list of those. Admittedly this could be worked around, but since this function already exists it makes sense to me it should be exposed.

I guess I can open a proposal but there is already one that includes that one function and some others, it turns out godotengine/godot-proposals#4854. And another one that is specifically about this functionality but under a different name godotengine/godot-proposals#3705

[Mickeon]

Documentation about this feature is entirely fine, if that was a bother.

[vaartis]

Rebased on master and fixed the conflicts.