Add MDX support to recommended VS Code extensions

This enables syntax highlighting in MDX documents.

Author: lpsinger

.vscode/extensions.json

@@ -6,7 +6,8 @@
   "recommendations": [
     "dbaeumer.vscode-eslint",
     "esbenp.prettier-vscode",
-    "ms-playwright.playwright"
+    "ms-playwright.playwright",
+    "unifiedjs.vscode-mdx"
   ],
   // List of extensions recommended by VS Code that should not be recommended for users of this workspace.
   "unwantedRecommendations": []