Revert "chore: remove @vant/markdown-loader package"

This reverts commit 6eec7f3c2f.

packages/vant-markdown-loader/src/card-wrapper.js

@@ -0,0 +1,16 @@
+module.exports = function cardWrapper(html) {
+  const group = html
+    .replace(/<h3/g, ':::<h3')
+    .replace(/<h2/g, ':::<h2')
+    .split(':::');
+
+  return group
+    .map(fragment => {
+      if (fragment.indexOf('<h3') !== -1) {
+        return `<div class="card">${fragment}</div>`;
+      }
+
+      return fragment;
+    })
+    .join('');
+};