`#[func]` support for variadic functions (vararg)

[kekobleibtkeko]

As suggested in #1471 , letting a #[func] appear as a vardiac function for flexibility, especially when used in conjunction with other APIs, such as Lua.

And the way I meant it was defining a #[func] that lets the gdscript side see it as a vararg function.

Ah, I see! Indeed, that's currently not possible -- but it's a good feature request, too.

Originally posted by @Bromeon in #1471

[Bromeon]

Vardiac arrest 💀

Could be something like:

#[func]
fn method(name: GString, #[vararg] args: VarArray) { ... }

or even:

#[func]
fn method(name: GString, args: Varargs) { 
    for v in args.iter() {
        // v has type &Variant
    }

    // Some convenience APIs like:
    let array = args.to_var_array();
    let elem_count = args.len();
    ...
}

I think we get the varargs as (*const Variant, usize) on the FFI size, so we can possibly avoid the allocation with a dedicated Varargs type. Then again, that might not matter if calls already downgrade to varcall (from ptrcall) when using varargs... would need to benchmark. Just some ideas 🧐

Prior art: gdnative::export::Varargs