Feat: add new Markdown parser (#39800)

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

tools/challenge-md-parser/mdx/plugins/utils/get-file-visitor.test.js

@@ -0,0 +1,33 @@
+describe('get-file-visitor', () => {
+  it('should join code with newlines', () => {
+    /* i.e. if you've got two js code blocks it should do this
+
+    ```js
+    one
+    ```
+
+    ```js
+    two
+    ```
+
+    become
+
+    ```js
+    one
+    two
+    ```
+
+    not
+
+    ```js
+    onetwo
+    ```
+    or
+    ```js
+
+    one
+    two
+    ```
+    */
+  });
+});