refactor: Move nix outside of .config dir

Technically not a config but a compose file, so I think it's better to
have outside of `.config` dir

nix/lib/mkCommon.nix

@@ -0,0 +1,21 @@
+{ pkgs, pkgs-unstable, ... }:
+
+let
+  custom = {
+    python = (pkgs.python312Full.withPackages (py: [
+      py.pip
+      py.tkinter
+      py.dnspython
+    ]));
+  };
+  custom.inkscape = if pkgs.stdenv.isDarwin then pkgs.casks.inkscape else (pkgs.inkscape.override { python3 = custom.python; });
+in {
+  inherit custom;
+  packages = [
+    pkgs.zsh
+    pkgs.home-manager
+    pkgs.zoxide
+    pkgs.ruby
+    pkgs._7zz
+  ];
+}