feat(emacs-pkgs): Introduce nix-util helper library

Adds an Emacs library with so far a grand total of one helper function
that can prefetch and insert a git repository at point.

This is very useful for the various Go repo imports I am doing at the
moment.

tools/emacs-pkgs/nix-util/default.nix

@@ -0,0 +1,7 @@
+{ pkgs, ... }:
+
+pkgs.third_party.emacsPackagesNg.trivialBuild rec {
+  pname = "nix-util";
+  version = "1.0";
+  src = ./nix-util.el;
+}