main: Change conditional to a switch

Prepares for next commit. No functional change.

highlighters/main/main-highlighter.zsh

@@ -848,14 +848,16 @@ _zsh_highlight_main_highlighter_highlight_argument()
   local -a match mbegin mend
   local MATCH; integer MBEGIN MEND
 
-  if [[ $arg[1] == - ]]; then
+  case "$arg[1]" in
+    '-')
       if [[ $arg[2] == - ]]; then
         base_style=double-hyphen-option
       else
         base_style=single-hyphen-option
       fi
       path_eligible=0
-  fi
+      ;;
+  esac
 
   for (( i = 1 ; i <= end_pos - start_pos ; i += 1 )); do
     case "$arg[$i]" in