[Feature Request] add demo of vue3-ts in vant-demo

[wjw-gavin]

这个功能解决了什么问题？

add demo of vue3-ts (script-setup)

你期望的 API 是什么样子的？

Hi, I really like vant-ui very much and I added vue3-ts demo to vant-demo, it's relatively simple so I just passed my own test, can you check it and merge it? (if there are no bugs) Thanks!

Here