Builds can be slow and the legacy debug adapter would handle Flutter's `app.progress` events to update the toast notification during builds. This was lost in the new adapters - we should only a single "Launching.." notification for the whole progress.

This change listens to `app.progress` events and forwards those with `finished=false` to the client if the launch progress is still active.

Fixes https://github.com/Dart-Code/Dart-Code/issues/4938

https://github.com/flutter/flutter/assets/1078012/8c60cf08-e034-4a72-b31e-9c61dca388bf