Feat: add new Markdown parser (#39800)

and change all the challenges to new `md` format.

tools/challenge-md-parser/mdx/plugins/utils/get-id.js

@@ -0,0 +1,8 @@
+// getId expects the image reference node to be the sole node in a paragraph
+function getId(node) {
+  const { type, name, attributes } = node;
+  if (type !== 'leafDirective' || name !== 'id' || !attributes) return null;
+  return attributes.id;
+}
+
+module.exports = getId;