init: lib/flake

A subflake that can be indidividually accessed without also providing
an interface to the whole of nixpkgs.

Usage:
inputs.nixpkgs-lib.url = "github:NixOS/nixpkgs?dir=lib"

lib/flake.nix

@@ -0,0 +1,5 @@
+{
+  description = "Library of low-level helper functions for nix expressions.";
+
+  outputs = { self }: { lib = import ./lib; };
+}