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/diffview.lua

@@ -0,0 +1,20 @@
+-- Easily cycle through diffs
+-- https://github.com/sindrets/diffview.nvim
+return {
+  "sindrets/diffview.nvim",
+  lazy = false,
+  dependencies = "nvim-lua/plenary.nvim",
+  keys = {
+    { "<leader>gd", "<cmd>DiffviewOpen<cr>", desc = "Diffview Open" },
+    { "<leader>gD", "<cmd>DiffviewClose<cr>", desc = "Diffview Close" },
+  },
+  opts = {
+    view = {
+      use_icons = true,
+      default = {
+        layout = "diff2_horizontal",
+        winbar_info = false,
+      },
+    },
+  },
+}