open_command: fix and improve command for WSL

- Add double quotes to command so that the next argument isn't
  interpreted as the title for the start command.

- If the first argument is a valid path, convert it to Windows path
  notation. If `wslpath` fails—because it's a path from inside WSL,
  which cannot be converted to Windows path notation— fail with an
  error code.

  This last circumstance will show an error like so:

    wslpath: path: Result not representable

lib/functions.zsh

@@ -22,9 +22,10 @@ function open_command() {
   case "$OSTYPE" in
     darwin*)  open_cmd='open' ;;
     cygwin*)  open_cmd='cygstart' ;;
-    linux*)   [[ $(uname -a) =~ "Microsoft" ]] && \
-                open_cmd='cmd.exe /c start' || \
-                open_cmd='xdg-open' ;;
+    linux*)   ! [[ $(uname -a) =~ "Microsoft" ]] && open_cmd='xdg-open' || {
+                open_cmd='cmd.exe /c start ""'
+                [[ -e "$1" ]] && { 1="$(wslpath -w "${1:a}")" || return 1 }
+              } ;;
     msys*)    open_cmd='start ""' ;;
     *)        echo "Platform $OSTYPE not supported"
               return 1