feat: add serve command

[kiaking]

This PR adds a new serve command inspired by Nuxt's start command. It will serve static files under .vitepress/dist folder generated by build command.

This is an easy way to test your production build files locally.

$ vitepress build # build docs first
$ vitepress serve # serve the built docs locally

Using Koa and Koa Static as the web server since it's already included with Vite.