feature/devServer.before

[PENG Rui]

https://vitejs.dev/guide/api-javascript.html#vitedevserver
https://webpack.js.org/configuration/dev-server/#devserverbefore

vite expose connectjs instance while webpack devServer expose expressjs instance
many use use vue.config.js#devServer#before to mount mock api server for development
how to do this in vite with compatibility ? maybe we should transform connect instance to express instance in vite#configureServer hook payload