feat(wpcarro/emacs): Package string.el

Mostly just a wrapper around s.el (for now?). Eventually I'd like to prune the
dependency on dash.el (and maybe s.el).

Change-Id: I5c2ba256524bedd93fcd13933fdbd95b1ddff6f8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6002
Reviewed-by: wpcarro <wpcarro@gmail.com>
Autosubmit: wpcarro <wpcarro@gmail.com>
Tested-by: BuildkiteCI

users/wpcarro/emacs/default.nix

@@ -29,6 +29,7 @@ let
       list
       maybe
       set
+      string
       struct
       zle
     ]) ++