ZST type for synthetic properties

[Bromeon]

We should implement a ZST type that can be used as a field, like gdnative's Property<T>.
This type will not store any value itself, but delegate to the custom getter and setter.

Example:

#[derive(GodotClass)]
struct MyClass {
    #[var(get)]
    synthetic_string: Property<GString>,
}

#[godot_api]
impl MyClass {
    #[func]
    fn get_synthetic_string(&self) -> GString {
        // Derive on-the-fly from instance state.
    }
}

I originally thought this might be interesting as a workaround for #[constant] which is limited to integers. However, properties make impractical constants as they need a pre-existing instance. Static functions are better suited.

[Yarwin]

Sounds cool.

I would propose calling it Placeholder<Type> instead. Property is something concrete in godot, so it might be a bit confusing.

Should getter/setter be obligatory to implement? If not, it would take care of some new/quirky PropertyFlags that are not handled by godot-rust yet – right now such examples could be group and categories (see: #226 (comment)) (not as a "real" supported solution ofc, more like band-aid before we figure out the real thing)