feat(nix): nix and back again

I tried Nix, but it had too many downsides so I removed it.

1. Didn't like that all files are immutable and simple config changes
need a complete rebuild.
2. Setting up a new Mac didn't work as smoothly as promised. Not worth
the effort.
3. It sucked a lot to always have to type in the password twice on each
darwin-rebuild
4. It solves problems I never had.

.config/nvim/lua/plugins/highlight-undo.lua

@@ -0,0 +1,15 @@
+-- highlight-undo.nvim
+-- https://github.com/tzachar/highlight-undo.nvim
+return {
+  "tzachar/highlight-undo.nvim",
+  config = function()
+    require("highlight-undo").setup({
+      hlgroup = "HighlightUndo",
+      duration = 300,
+      keymaps = {
+        { "n", "u", "undo", {} },
+        { "n", "<C-r>", "redo", {} },
+      },
+    })
+  end,
+}