Add support for indexed properties

[mihe]

Fixes #1182.
Requires godotengine/godot#79763.

This adds support for binding indexed properties from godot-cpp, which at the moment is not possible due to the p_index parameter in ClassDB::add_property being discarded completely.

This PR resolves this by passing p_index to the new GDExtension function classdb_register_extension_class_property_indexed, which is effectively the same as the old classdb_register_extension_class_property function but with the added p_index parameter.

I also snuck in the removal of PropertySetGet, which seems to be some old unused remnant from a distant past.

[dsnopek]

Thanks!

Can you copy the gdextension_interface.h changes from your Godot PR, into here as well?

Otherwise, the code looks great and works perfectly in my testing :-)

[dsnopek]

Thanks!

This won't pass tests on CI until after the Godot PR is merged, but since I've tested it locally I feel fine approving it. (We'll do a CI run just before merging anyway.)

[mihe]

I forgot to mention this, but note that this does technically mean that the old classdb_register_extension_class_property function is now unused in godot-cpp, so I'm not sure whether the whole LOAD_PROC_ADDRESS dance for that one should be removed perhaps.

Just give me the word and I'll yank it.

[dsnopek]

No, I think we should leave it in. I doubt anyone is using that particular function, but I have seen folks directly calling some of the gdextension_interface_* functions, so I think we should load them all.

[dsnopek]

Godot PR merged, and the tests here are now passing, so merging.

Thanks!