typeahead: Replace code block language hint text with more clear options

This commit
- Replace the blank option with an italicized option that's the current
   default language, if there is one selected with "default" label.
- Make the "text" option more informative by adding (no highlighting)
   to the label.
- Remove the hint for "text".
- Prioritize as left to right, before start typing:
   blank/default language, text, quote, spoiler, math, everything else...

fixes: #33682

web/styles/zulip.css

@@ -2349,6 +2349,7 @@ body:not(.spectator-view) {
     }
 }
 
+.default-language-display,
 .empty-topic-display,
 .empty-topic-placeholder-display::placeholder {
     font-style: italic;