chore(nif): add nif stubs

Signed-off-by: SphericalKat <amolele@gmail.com>

lib/ketbin/utils/syntax.ex

@@ -0,0 +1,6 @@
+defmodule Ketbin.Utils.Syntax do
+  use Rustler, otp_app: :ketbin, crate: "ketbin_utils_syntax"
+
+  # When your NIF is loaded, it will override this function.
+  def add(_a, _b), do: :erlang.nif_error(:nif_not_loaded)
+end