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/darwin/libFixup.nix

@@ -0,0 +1,10 @@
+{ package, runCommandLocal, name }:
+
+runCommandLocal "${name}-symlink" {}
+  ''
+  dest="/usr/local/opt/${name}"
+
+  mkdir -p $dest
+
+  ln -sf "${package.out}/lib/" "$dest" && mkdir -p "$out" && touch "$out/${name}-done"
+  ''

nix/lib/darwin/mkApp.nix

@@ -0,0 +1,22 @@
+# REF: https://github.com/bandithedoge/nixpkgs-firefox-darwin/blob/ceaca2359e5371ccef3f92a36baf7c438b354afb/overlay.nix
+{ stdenv, undmg, _7zz, pname, version, sourceRoot ? ".", appFileName, src, meta
+, nativeBuildInputs ? [ undmg _7zz ]
+, unpackPhase ? "undmg $src || 7zz x -snld $src"
+, installPhase ? ''
+    mkdir -p $out/Applications
+    cp -R ${appFileName} "$out/Applications/"
+''
+}:
+
+stdenv.mkDerivation rec {
+  inherit pname version;
+
+  inherit src nativeBuildInputs;
+
+  inherit sourceRoot;
+
+  phases = [ "unpackPhase" "installPhase" ];
+  inherit unpackPhase installPhase;
+
+  inherit meta;
+}

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
+  ];
+}

nix/lib/mkSystem.nix

@@ -0,0 +1,29 @@
+{ arch, stable, unstable, nur, extraOverlays ? [] }:
+
+let
+  overlays = [
+      (import ../overlays/python.nix)
+  ] ++ extraOverlays;
+
+  # Placed here so that we don't need to specify hash for fetchTarball
+  packageOverrides = pkgs: {
+    nur = import nur {
+      nurpkgs = pkgs;
+      inherit pkgs;
+    };
+  };
+in {
+  system = arch;
+  pkgs = import stable {
+    system = arch;
+    inherit overlays;
+    config.allowUnfree = true;
+    config.packageOverrides = packageOverrides;
+  };
+  pkgs-unstable = import unstable {
+    system = arch;
+    inherit overlays;
+    config.allowUnfree = true;
+    # config.packageOverrides = packageOverrides;
+  };
+}