feat: extended nixpkgs-lib template

nixpkgs-extend-lib/flake.nix

···
+{
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";
+  };
+
+  outputs = inputs: {
+    lib = import ./lib { inherit inputs; };
+  };
+}

nixpkgs-extend-lib/lib/default.nix

···
+# modified from
+#  https://github.com/isabelroses/dotfiles/blob/c4b76ef3db97976ba7a26127d38cae7a11c056e9/modules/flake/lib/default.nix
+#
+# following https://github.com/NixOS/nixpkgs/blob/77ee426a4da240c1df7e11f48ac6243e0890f03e/lib/default.nix
+# as a rough template we can create our own extensible lib and expose it to the flake
+# we can then use that elsewhere like our hosts
+{ inputs, ... }:
+let
+  nixpkgsLib = inputs.nixpkgs.lib;
+
+  myLib = nixpkgsLib.makeExtensible (
+    self:
+    let
+      callLib = file: import file { lib = self; };
+    in
+    {
+      math = callLib ./math.nix;
+
+      # reexport from our math namespace
+      inherit (self.math) increment;
+    }
+  );
+
+  # we need to extend myLib with the nixpkgs lib to get the full set of functions
+  # if we do it the otherway around we will get errors saying mkMerge and so on don't exist
+  finalLib = myLib.extend (_: _: nixpkgsLib);
+in
+finalLib

nixpkgs-extend-lib/lib/math.nix

···
+{ lib }:
+{
+  increment = x: lib.add x 1;
+}