[Demo] Local-only supabase todo app

[Chriztiaan]

This adds a demo project that aims to demonstrate how an app could be built starting from a local-only state and later being "upgraded" to a version that has sync capabilities. It started off as a copy of the existing todo app that was stripped of some of the features. The biggest changes are in powersync.dart, login/signup views, and models/schema.dart (which illustrates how to update the schema to supporting syncing).

The readme has a basic description of how everything works, with a small mermaid diagram to show how the app switches from the local-only mode to sync mode.