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-chat.lua

@@ -0,0 +1,13 @@
+-- Chat with GitHub Copilot in Neovim
+-- https://github.com/CopilotC-Nvim/CopilotChat.nvim
+return {
+  {
+    "CopilotC-Nvim/CopilotChat.nvim",
+    branch = "canary",
+    dependencies = {
+      { "zbirenbaum/copilot.lua" }, -- or github/copilot.vim
+      { "nvim-lua/plenary.nvim" }, -- for curl, log wrapper
+    },
+    opts = {},
+  },
+}