Feat: add new Markdown parser (#39800)

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

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

@@ -0,0 +1,27 @@
+const { isEmpty } = require('lodash');
+const getAllBetween = require('./utils/between-headings');
+const mdastToHTML = require('./utils/mdast-to-html');
+
+function addText(sectionIds) {
+  if (!sectionIds || !Array.isArray(sectionIds) || sectionIds.length <= 0) {
+    throw new Error('addText must have an array of section ids supplied');
+  }
+  function transformer(tree, file) {
+    for (const sectionId of sectionIds) {
+      const textNodes = getAllBetween(tree, `--${sectionId}--`);
+      const sectionText = mdastToHTML(textNodes);
+
+      if (!isEmpty(sectionText)) {
+        file.data = {
+          ...file.data,
+          [sectionId]: `<section id="${sectionId}">
+${sectionText}
+</section>`
+        };
+      }
+    }
+  }
+  return transformer;
+}
+
+module.exports = addText;