Add support for classes from Godot core API dump

[Xrayez]

Currently only a handful of core types are supported by the generator natively: https://github.com/Xrayez/gdmodule/blob/0f39e947ba2897d459a3743ddb7d22d44e1c4d29/gdtypes.py#L26-L34

It would be nice for the generator to optionally dump Godot core API so that more versatile classes could be generated.

I suppose GDNative does something similar out of the box, see command line --help:

--gdnative-generate-json-api Generate JSON dump of the Godot API for GDNative bindings.

But it seems like it's general-purpose enough.