Don't add language- to class if already prefixed

See commonmark/commonmark.js@c7aef33

Author: colinodell

CHANGELOG.md

@@ -9,6 +9,7 @@ Updates should follow the [Keep a CHANGELOG](https://keepachangelog.com/) princi
 ### Fixed
 
 - Fixed declaration parser being too strict
+- `FencedCodeRenderer`: don't add `language-` to class if already prefixed
 
 ## [2.4.1] - 2023-08-30
 

src/Extension/CommonMark/Renderer/Block/FencedCodeRenderer.php

@@ -41,7 +41,11 @@ public function render(Node $node, ChildNodeRendererInterface $childRenderer): \
 
         $infoWords = $node->getInfoWords();
         if (\count($infoWords) !== 0 && $infoWords[0] !== '') {
-            $attrs->append('class', 'language-' . $infoWords[0]);
+            $class = $infoWords[0];
+            if (!\str_starts_with($class, 'language-')) {
+                $class = 'language-' . $class;
+            }
+            $attrs->append('class', $class);
         }
 
         return new HtmlElement(