FIX: bypass highlighths for long code in auto mode (#28399)

Long code is considered 1000 characters atm. 10000+ characters would take seconds due to how highlightjs will consider every installed lang before choosing the best candidate.

app/assets/javascripts/discourse/app/lib/highlight-syntax.js

@@ -42,6 +42,10 @@ export default async function highlightSyntax(elem, siteSettings, session) {
       }
     }
 
+    if (lang === "auto" && e.innerHTML.length > 1000) {
+      return;
+    }
+
     const canHighlight = lang && (lang === "auto" || hljs.getLanguage(lang));
 
     if (canHighlight) {