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/fs.fish

@@ -0,0 +1,13 @@
+function fs --description "Determine size of a file or total size of a directory"
+    if du -b /dev/null >/dev/null 2>&1
+        set arg -sbh
+    else
+        set arg -sh
+    end
+
+    if test -n "$argv"
+        du $arg -- $argv
+    else
+        du $arg .[^.]* *
+    end
+end