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/gh-line.lua

@@ -0,0 +1,9 @@
+-- Open the link of current line on GitHub
+-- https://github.com/ruanyl/vim-gh-line
+-- <leader>gh (line) or <leader>gb (blame)
+return {
+  "ruanyl/vim-gh-line",
+  config = function()
+    vim.g.gh_github_domain = "source.xing.com"
+  end,
+}