Feat: add new Markdown parser (#39800)

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

curriculum/challenges/english/10-coding-interview-prep/rosetta-code/averages-root-mean-square.md

@@ -1,73 +1,56 @@
 ---
-title: Averages/Root mean square
 id: 594da033de4190850b893874
+title: Averages/Root mean square
 challengeType: 5
 forumTopicId: 302228
 ---
 
-## Description
-<section id='description'>
-Compute the  <a href="https://en.wikipedia.org/wiki/Root mean square" title="wp: Root mean square" target='_blank'>Root mean square</a>  of the numbers 1 through 10 inclusive.
-The <i>root mean square</i> is also known by its initials RMS (or rms), and as the <strong>quadratic mean</strong>.
+# --description--
+
+Compute the [Root mean square](<https://en.wikipedia.org/wiki/Root mean square> "wp: Root mean square") of the numbers 1 through 10 inclusive.
+
+The *root mean square* is also known by its initials RMS (or rms), and as the **quadratic mean**.
+
 The RMS is calculated as the mean of the squares of the numbers, square-rooted:
-<big>$$x_{\mathrm{rms}} = \sqrt {{{x_1}^2 + {x_2}^2 + \cdots + {x_n}^2} \over n}. $$</big>
-</section>
 
-## Instructions
-<section id='instructions'>
+$$x\_{\\mathrm{rms}} = \\sqrt {{{x_1}^2 + {x_2}^2 + \\cdots + {x_n}^2} \\over n}. $$
 
-</section>
+# --hints--
 
-## Tests
-<section id='tests'>
-
-```yml
-tests:
-  - text: <code>rms</code> should be a function.
-    testString: assert(typeof rms === 'function');
-  - text: <code>rms([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])</code> should equal <code>6.2048368229954285</code>.
-    testString: assert.equal(rms(arr1), answer1);
-
-```
-
-</section>
-
-## Challenge Seed
-<section id='challengeSeed'>
-
-<div id='js-seed'>
+`rms` should be a function.
 
 ```js
-function rms(arr) {
-
-}
+assert(typeof rms === 'function');
 ```
 
-</div>
+`rms([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])` should equal `6.2048368229954285`.
 
+```js
+assert.equal(rms(arr1), answer1);
+```
 
-### After Test
-<div id='js-teardown'>
+# --seed--
+
+## --after-user-code--
 
 ```js
 const arr1 = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
 const answer1 = 6.2048368229954285;
 ```
 
-</div>
+## --seed-contents--
 
-</section>
+```js
+function rms(arr) {
 
-## Solution
-<section id='solution'>
+}
+```
 
+# --solutions--
 
 ```js
 function rms(arr) {
   const sumOfSquares = arr.reduce((s, x) => s + x * x, 0);
   return Math.sqrt(sumOfSquares / arr.length);
 }
-
 ```
-
-</section>