Expose Object::free_instance_binding() to GDExtension

[CedNaru]

I noticed that the latest Godot 4.2 beta added a new free_instance_binding function to Object.
It is only exposed to the Editor but it has uses for regular GDExtension memory management.

We would welcome that change for our Godot Kotlin binding as it makes some memory management operations easier.
One of the use cases is for example to free the JVM instance used to wrap the native object pointer when it's no longer used in the managed side of the memory. That way we don't have to wait for the native object freed to release the Kotlin wrapper as our instance binding hold a reference to the JVM instance (so far we had to convert it to a weak reference instead of removing the binding).

[dsnopek]

This use case makes sense to me! The code looks good - this is basically just exposing functionality that was already there, so I think it's pretty safe.

I don't know if the production team will be OK with merging this for Godot 4.2, given that the feature freeze is imminent, but if it doesn't make that cut off, this can be an early 4.3 addition.

[akien-mga]

Thanks!