chore: Add a recommended extension for .mdx files (zmkfirmware#2381)

Author: nmunnich

.vscode/extensions.json

@@ -4,6 +4,7 @@
     "ms-python.python",
     "ms-vscode.cpptools",
     "plorefice.devicetree",
-    "twxs.cmake"
+    "twxs.cmake",
+    "unifiedjs.vscode-mdx"
   ]
 }