home: add direnv

This uses nixos-community's version of `use_nix` which allows for
persistent shells, and is now compatible with flakes.

home/default.nix

@@ -1,6 +1,7 @@
 { ... }:
 {
   imports = [
+    ./direnv.nix
     ./zsh
   ];
 }

home/direnv.nix

@@ -0,0 +1,8 @@
+{ ... }:
+{
+  programs.direnv = {
+    enable = true;
+    # A better `use_nix`
+    enableNixDirenvIntegration = true;
+  };
+}