File sync between server & client

A feature enables file sync between server and client. When add / remove assets/ directory files, the change will be notificated to client through WebSocket connection. On the client app, interpreting the notification, the main.ts logics fetch new resources and remove old caches.

warning

This implementation is incomplete. It can understand add / remove files, but not modify files. If you want to execute it correctly, you need to add a unique version hash to distributed files.