fix: prevent grouping consecutive code blocks with the same language (#667)

* fix: prevent grouping consecutive code blocks with the same language

Two consecutive code blocks with the same language (e.g., both `mjs`)
were incorrectly grouped into a CJS/MJS switchable tab. Add a check
to ensure the two code blocks have different languages before creating
a switchable code tab.

Refs: nodejs/node#62181

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix: add optional chaining for defensive null safety

* fix: update to assume [0] and [1] always exist

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: okuryu

src/utils/highlighter.mjs

@@ -142,8 +142,12 @@ export default function rehypeShikiji() {
         currentIndex += 1;
 
         // Since we only support CJS/MJS switch, we should have exactly 2 elements
-        // in order to create a switchable code tab
-        if (codeElements.length === 2) {
+        // with different languages in order to create a switchable code tab
+        if (
+          codeElements.length === 2 &&
+          codeElements[0].properties?.language !==
+            codeElements[1].properties?.language
+        ) {
           const switchablePreElement = createElement(
             'pre',
             {