direnv: enable nushell integration

This enables nushell integration by default for direnv, similar to
bash/zsh/fish. The slightly verbose way of setting this is to ensure
that peoples' existing nushell configuration isn't overwritten, only
appended to, as would be the case if we just used the integration
example from the nushell docs:

  https://www.nushell.sh/cookbook/direnv.html

Closes #3520

tests/modules/programs/direnv/default.nix

@@ -1,6 +1,7 @@
 {
   direnv-bash = ./bash.nix;
   direnv-nix-direnv = ./nix-direnv.nix;
+  direnv-nushell = ./nushell.nix;
   direnv-stdlib = ./stdlib.nix;
   direnv-stdlib-and-nix-direnv = ./stdlib-and-nix-direnv.nix;
 }

tests/modules/programs/direnv/nushell.nix

@@ -0,0 +1,19 @@
+{ pkgs, ... }:
+
+{
+  programs.nushell.enable = true;
+  programs.direnv.enable = true;
+
+  test.stubs.nushell = { };
+
+  nmt.script = let
+    configFile = if pkgs.stdenv.isDarwin then
+      "home-files/Library/Application Support/nushell/config.nu"
+    else
+      "home-files/.config/nushell/config.nu";
+  in ''
+    assertFileExists "${configFile}"
+    assertFileRegex "${configFile}" \
+      'let direnv = (/nix/store/.*direnv.*/bin/direnv export json | from json)'
+  '';
+}