Refactoring this project

When I created this project, it was literally a morning's project. I woke up with the idea, and at noon I was making the first release.

Looking at this code, it seems a little confusing, so I decided to move all server logic to the `nano_server` package, and create just a thin layer of compatibility with the Flutter-inspired interface.
This will make the code cleaner, faster, and easier to debug.