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.

.gitignore

@@ -1,2 +1,14 @@
 # Karabiner backups
 automatic_backups
+
+# Homebrew lock files
+homebrew/*.lock.json
+
+# Tmux Plugin Manager
+.config/tmux/plugins/
+
+# Fish
+.config/fish/completions
+.config/fish/conf.d
+.config/fish/fish_variables
+.config/fish/functions/*