Final and non-instantiable classes

[Bromeon]

Final classes cannot be inherited from. Attempting to do so will be met with a compile error:

#[derive(GodotClass)]
#[class(init, base=Os)]
struct MyClass {}

error: Class `Os` is final and cannot be inherited from.

Non-instantiable classes cannot be default-constructed; i.e. the NewGd/NewAlloc traits aren't implemented for them.
Examples here are all the tweeners.

Closes #1129.

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-1162