Bigs cannot be initialized with another Big

[NumbuhFour]

If you try to do Big.new(another_big) to copy a Big, it fails to copy over the original's values.

The _init function of Big checks m.is_class("Big") but per godotengine/godot#21789 this fails.
The return of m.get_class() does not return "Big", but returns the native class "RefCounted", so an alternative solution is required.

Repro steps:

var some_big:Big = Big.new(99)
var copy_big:Big = Big.new(some_big)

Expected: copy_big's value should be 99
Result: copy_big's value is 0

This was found in Godot4 with source pulled from both godot and GodotBigNumberClass pulled today

[ChronoDK]

Thanks, I have fixed it right away!