feat(nvim): add hunk.nvim plugin

This should be a better diff editor for `jj`

modules/nixvim/dev-plugins.nix

@@ -2,10 +2,11 @@
   lib,
   pkgs,
   config,
-  helpers,
   ...
 }:
 let
+  helpers = if lib.hasAttrByPath [ "lib" "nixvim" ] config then config.lib.nixvim else lib.nixvim;
+
   inherit (helpers) enableExceptInTests;
   inherit (lib.trivial) const;
   cfg = config.jhome.nvim;
@@ -138,7 +139,7 @@ in
       {
         extraPackages = [ (pkgs.python3.withPackages (p: [ p.jupytext ])) ];
         plugins = {
-          image.enable = enableExceptInTests;
+          image.enable = lib.mkForce enableExceptInTests;
           jupytext = {
             enable = true;
             settings.custom_language_formatting.python = {