Deep copies taking into account Rust state (unlike `Node::duplicate()`)

Taking the docs as an example,
```rust
#[derive(GodotClass)]
#[derive(GodotClass)]
#[class(base=Node3D)]
struct Monster {
name: String,
hitpoints: i32,
base: Base,
}
```

Calling .duplicate() on an instance will correctly copy all the stuff that Godot knows about, but name and hitpoints will be their .init() values

It makes sense that it works this way to a certain degree, since duplicate() is a Godot engine thing, so .duplicate() just calls the Godot function without doing anything special for the gdext portions, but it was surprising to me, and may be to others

There should be a way to fully clone a node and its children including their Rust properties, without requiring that each Rust property is `#[var]`-compatible.

original Discord thread: https://discord.com/channels/723850269347283004/1236100168722681927