Feat: add new Markdown parser (#39800)

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

tools/challenge-md-parser/mdx/plugins/add-frontmatter.js

@@ -0,0 +1,18 @@
+const visit = require('unist-util-visit');
+const YAML = require('js-yaml');
+
+function plugin() {
+  return transformer;
+
+  function transformer(tree, file) {
+    visit(tree, 'yaml', visitor);
+
+    function visitor(node) {
+      const frontmatter = YAML.load(node.value);
+
+      file.data = { ...file.data, ...frontmatter };
+    }
+  }
+}
+
+module.exports = plugin;