#[func(virtual)] now supports async fn for GDScript coroutines

[Bromeon]

Proof-of-concept for

#[func(virtual)]
async fn script_virtual_fn(&mut self) { ... }

which is wired up with godot-rust's async executor and can call GDScript coroutines.
Closes #1640 from @fredericvauchelles.

PR will need some cleanup. I also have plans to generalize this method to allow reflection-style call_async() to arbitrary GDScript functions, not just script-virtual ones.

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-1645