Add rule regarding use of Void outside C lib bindings

[nobodywasishere]

crystal-lang/crystal#10657

getter qux : Void?
           # ^^^^ error: `Void` should only be used in the context of C bindings

def foo(bar : Void) : Void
            # ^^^^ error: `Void` should only be used in the context of C bindings
                    # ^^^^ error: `Void` should only be used in the context of C bindings
  baz = uninitialized Void
                    # ^^^^ error: `Void` should only be used in the context of C bindings
end

[Sija]

I like the idea, just mind you that usages of Pointer(Void) should still be allowed.