Feat: add new Markdown parser (#39800)

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

tools/challenge-md-parser/mdx/plugins/utils/mdast-to-html.js

@@ -0,0 +1,15 @@
+const hastToHTML = require('hast-util-to-html');
+const mdastToHast = require('mdast-util-to-hast');
+const { root } = require('mdast-builder');
+
+function mdastToHTML(nodes) {
+  if (!Array.isArray(nodes))
+    throw Error('mdastToHTML expects an array argument');
+  // - the 'nodes' are children, so first need embedding in a parent
+
+  return hastToHTML(mdastToHast(root(nodes), { allowDangerousHtml: true }), {
+    allowDangerousHtml: true
+  });
+}
+
+module.exports = mdastToHTML;