feat: support default script tag

[lq782655835]

problem

It will throw a error when i ignore script tag，because of scirpt tag is unnecessary in my markdown docs. like below：

solution

Pull Request：https://github.com/xiguaxigua/vuepress-plugin-demo-block/pull/23

[calebman]

Try this plugin https://github.com/calebman/vuepress-plugin-demo-container?

Good support for examples.

[zch233]

@lq782655835 请教一下你这个 u-input 是怎么引入的