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/fish/functions/fwt.fish

@@ -0,0 +1,3 @@
+function fwt --description "Jump to Git worktree directory"
+    cd (git worktree list | awk '{print $1}' | fzf)
+end