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

This reverts commit 6eec7f3c2f.

packages/vant-markdown-loader/src/highlight.js

@@ -0,0 +1,10 @@
+const hljs = require('highlight.js');
+
+module.exports = function highlight(str, lang) {
+  if (lang && hljs.getLanguage(lang)) {
+    // https://github.com/highlightjs/highlight.js/issues/2277
+    return hljs.highlight(str, { language: lang, ignoreIllegals: true }).value;
+  }
+
+  return '';
+};