Feat: Create `Flutter` Stopwatch Tutorial

[nelsonic]

One of the "core" features of our App is timers, see: mvp.fly.dev 📱
At its' core the timer feature is a stopwatch. ⏱️

Todo

• Create a Flutter tutorial similar to: https://github.com/dwyl/phoenix-liveview-stopwatch
• Take the reader from zero to working App on a Mobile Device in as much detail as necessary
• Bonus Level: Persistence e.g: using drift dwyl/learn-flutter#70 to have a history of the timers you have captured on the device.

[LuchoTurtle]

The referenced PR #2 should close this issue and I think it's completed. The project works as "advertised" and the README should thoroughly explain how to get the app working 😄