main: Add highlighting for hashed directories with autocd and cdablevars

[zaidhaan]

See #930 (or at least just #930 (comment)).

What this PR does:

• If a hashed directory foo exists and the path it resolves to also exists, it will highlight it as a hashed-command
  

What this PR does not do:

• Does not highlight after further Tab-complete expansion of the hashed directory, even though it should be valid
  
  

If this feels like enough to be a PR on its own, then you could merge. If it feels incomplete, but on the right path, someone else could offer guidance and I'll see if I can complete it or they could pick it up themselves, or if it's completely wrong then it could just be closed.

[phy1729]

I think we can take the CDABLE_VARS documentation fairly literally

          If the argument to a cd command (or an implied cd with the
          AUTO_CD option set) is not a directory, and does not begin with
          a slash, try to expand the expression as if it were preceded by
          a `~' (see the section `Filename Expansion').

and just call _zsh_highlight_main_highlighter_expand_path again, but prefixed with a tilde. Something like

    elif (( autocd && cdablevars )) && [[ ${1[1]} != / ]]; then
      _zsh_highlight_main_highlighter_expand_path "~$1"
      if [[ -d $REPLY && -x $REPLY ]]; then
        REPLY=autodirectory
        return 0
      fi

That covers the case where the path continues and the case where a username is provided.