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/copilot.lua

@@ -0,0 +1,10 @@
+-- Copilot replacement
+-- https://github.com/zbirenbaum/copilot.lua
+-- https://github.com/zbirenbaum/copilot-cmp
+return {
+  "zbirenbaum/copilot-cmp",
+  dependencies = "zbirenbaum/copilot.lua",
+  opts = function()
+    require("copilot").setup()
+  end,
+}