Perf: optimize parameter check

Author: romkatv

highlighters/main/main-highlighter.zsh

@@ -750,7 +750,8 @@ _zsh_highlight_main_highlighter_highlight_list()
     fi
 
     # Expand parameters.
-    if (( ! in_param )) && _zsh_highlight_main_highlighter__try_expand_parameter "$arg"; then
+    if (( ! in_param )) && [[ $arg == \$* ]] &&
+       _zsh_highlight_main_highlighter__try_expand_parameter "$arg"; then
       # That's not entirely correct --- if the parameter's value happens to be a reserved
       # word, the parameter expansion will be highlighted as a reserved word --- but that
       # incorrectness is outweighed by the usability improvement of permitting the use of