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/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;
+}