Variadic functions

Godot 4.5 added Variadic functions to gdscript. I know we can call vararg stuff, but im unable to author a rust function to be a vararg function to be exposed to gdscript.

Start by reviewing the book's existing guidance on calling variadic GDScript functions and exposing Rust functions, then compare it with Godot 4.5's variadic-function behavior. Done means the documentation clearly explains whether and how a Rust function can be exposed as variadic, including a usable example or an explicit limitation.

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