Feat: add new Markdown parser (#39800)

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

tools/challenge-md-parser/mdx/plugins/utils/split-on-thematic-break.js

@@ -0,0 +1,20 @@
+const is = require('unist-util-is');
+
+// TODO: specific tests for this would be nice, even though it is somewhat
+// covered by the plugins that use it.
+function splitOnThematicBreak(nodes) {
+  return nodes.reduce(
+    (prev, curr) => {
+      if (is(curr, 'thematicBreak')) {
+        return [...prev, []];
+      } else {
+        const first = prev.slice(0, -1);
+        const last = prev.slice(-1)[0];
+        return [...first, [...last, curr]];
+      }
+    },
+    [[]]
+  );
+}
+
+exports.splitOnThematicBreak = splitOnThematicBreak;