Simplify GC leveraging `objectID` and `validatorCount`

[chippmann]

Godot 4 introduced a more sophisticated objectID containing many informations about an object. Such as a bit flag for it it's a reference, or a pointer to a slot object containing usefule information for us such as the actual pointer to the object and a validatorCount for the object index.

With this new information and the changes in https://github.com/utopia-rise/godot-kotlin-jvm/issues/306, we should be able to greatly simplify the GC code.