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

This reverts commit 6eec7f3c2f.

packages/vant-markdown-loader/src/md-parser.js

@@ -0,0 +1,14 @@
+const MarkdownIt = require('markdown-it');
+const markdownItAnchor = require('markdown-it-anchor');
+const highlight = require('./highlight');
+const { slugify } = require('transliteration');
+
+const parser = new MarkdownIt({
+  html: true,
+  highlight,
+}).use(markdownItAnchor, {
+  level: 2,
+  slugify,
+});
+
+module.exports = parser;