Omit return_value in gdextension_interface.json for void functions

[dsnopek]

Currently, the gdextension_interface.json requires specifying return_value for all functions, using the "type": "void" for ones without return values.

However, this is different than extension_api.json which omits the return_value in this case.

Per @Bromeon's suggestion on godotengine/godot-docs#11551 (comment), this updates the gdextension_interface.json to match extension_api.json

To ensure consistency, this also disallows using void without the pointer modifier as a type, so "type": "void" would now generate an error

[akien-mga]

Seems good to me.

[Repiteo]

Thanks!