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

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](https://github.com/godot-rust/gdext/issues/1471#issuecomment-3729018448)_