targets.darwin: add module

Currently, this module makes sure that `/Applications` directories for
packages in `home.packages` get linked into the user's environment.

tests/modules/targets-darwin/darwin.nix

@@ -0,0 +1,20 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+
+  darwinTestApp = pkgs.runCommandLocal "target-darwin-example-app" { } ''
+    mkdir -p $out/Applications
+    touch $out/Applications/example-app
+  '';
+
+in {
+  config = {
+    home.packages = [ darwinTestApp ];
+
+    nmt.script = ''
+      assertFileExists 'home-files/Applications/Home Manager Apps/example-app'
+    '';
+  };
+}

tests/modules/targets-darwin/default.nix

@@ -0,0 +1 @@
+{ targets-darwin = ./darwin.nix; }