add dev flake pattern

This allows us to have dev dependencies which are not propagated to the user.

devFlake/update-private-narhash

@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+# Used to update the private dev flake hash reference.
+set -euo pipefail
+
+cd "$(dirname "$0")"
+
+echo "Updating $PWD/private.narHash" >&2
+
+nix --extra-experimental-features 'flakes nix-command' flake lock ./private
+nix --extra-experimental-features 'flakes nix-command' hash path ./private >./private.narHash
+
+echo OK