* Ability to create script languages from GDExtension
* Some additions to gdnative_extension.h to make this happen
* Moved the GDExtension binder to core
This now allows creating scripting languages from GDExtension, with the same ease as if it was a module. It replaces the old PluginScript from Godot 3.x.
Warning: GodotCPP will need to be updated to support this (it may be a bit of work as ScriptInstance needs to be created over there again).
* Deprecates GDNative in favor of a simpler, lower level interface.
* New extension system allows registering core engine classes.
* Simple header interface in gdnative_interace.h
