Godot 4.x support

[marcello505]

At the moment of writing it's still unknown when Godot 4.0 will be released, however plenty of details about it have been shared, one of which is the complete overhaul of the GDNative system. Because of this it's highly likely that some major changes need to be introduced to support the 4.0 version, some of which will probably break 3.x support. Which is why it's probably for the best that once this happens, a legacy 3.x branch gets made that uses the older GDNative format. The fact that Godot is preparing to keep supporting 3.x even with the release of 4.x just shows how big of a change 4.0 will be.

It's unlikely that i will be able to work on 4.x support anytime soon since i'm dependant on the Godot-rust project to support it first. But it's good to plan things out in advance before i get to this stage.