overlays: add 'gruvbox-nvim-better-diff'

I dislike the new style of diff [1].

After somebody wrote a patch [2] I finally started experimenting with
what looked best to me.

This is using the old vibrant colours, which I like better. And avoids
using `reverse = true` to not break high-lighting during visual
selection.

This is using an overlay as it is _much_ easier to refer to the internal
colours in a `dark`/`light` agnostic way that way instead of the
intended "use the palette way" (due to breaking changes in [3] which,
incidentally, is the MR which changed diff high-lighting).

[1]: https://github.com/ellisonleao/gruvbox.nvim/issues/290
[2]: https://github.com/ellisonleao/gruvbox.nvim/pull/291
[3]: https://github.com/ellisonleao/gruvbox.nvim/pull/280

overlays/gruvbox-nvim-better-diff/default.nix

@@ -0,0 +1,4 @@
+self: prev:
+{
+  vimPlugins = prev.vimPlugins.extend (self.callPackage ./generated.nix { });
+}