Feat: add new Markdown parser (#39800)

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

curriculum/challenges/english/10-coding-interview-prep/project-euler/problem-112-bouncy-numbers.md

@@ -1,41 +1,35 @@
 ---
 id: 5900f3dd1000cf542c50feef
-challengeType: 5
 title: 'Problem 112: Bouncy numbers'
+challengeType: 5
 forumTopicId: 301738
 ---
 
-## Description
-<section id='description'>
+# --description--
+
 Working from left-to-right if no digit is exceeded by the digit to its left it is called an increasing number; for example, 134468.
+
 Similarly if no digit is exceeded by the digit to its right it is called a decreasing number; for example, 66420.
+
 We shall call a positive integer that is neither increasing nor decreasing a "bouncy" number; for example, 155349.
+
 Clearly there cannot be any bouncy numbers below one-hundred, but just over half of the numbers below one-thousand (525) are bouncy. In fact, the least number for which the proportion of bouncy numbers first reaches 50% is 538.
+
 Surprisingly, bouncy numbers become more and more common and by the time we reach 21780 the proportion of bouncy numbers is equal to 90%.
+
 Find the least number for which the proportion of bouncy numbers is exactly 99%.
-</section>
 
-## Instructions
-<section id='instructions'>
+# --hints--
 
-</section>
-
-## Tests
-<section id='tests'>
-
-```yml
-tests:
-  - text: <code>euler112()</code> should return 1587000.
-    testString: assert.strictEqual(euler112(), 1587000);
+`euler112()` should return 1587000.
 
+```js
+assert.strictEqual(euler112(), 1587000);
 ```
 
-</section>
+# --seed--
 
-## Challenge Seed
-<section id='challengeSeed'>
-
-<div id='js-seed'>
+## --seed-contents--
 
 ```js
 function euler112() {
@@ -46,17 +40,8 @@ function euler112() {
 euler112();
 ```
 
-</div>
-
-
-
-</section>
-
-## Solution
-<section id='solution'>
+# --solutions--
 
 ```js
 // solution required
 ```
-
-</section>