update states_rebuilder sample to use the latest version and to use immutable state

[GIfatahTH]

This updates the states_rebuilder implementation to use the 1.15.0 version.

Changes:

• Immutable state: Now Todo and TodosState are immutable objects.
• methods in TodosState are static pure functions
• New requirement: I add the requirement that if a todo is added, deleted or updated, the modification is instantly reflected in the UI and the async savaTodos is executed in the background. If the saveTodos fails and throws an error, the last state is restored and a SnackBar containing the error is displayed.
  This requirement does not cost me to add more than a few lines of code for all the possible add, delete, and update.
• Widget test: almost all the widget behavior is tested. Even the added requirement is tested.

[GIfatahTH]

The test fails because :
every version of flutter_test from sdk depends on collection 1.14.11 and states_rebuilder >=2.0.0 depends on collection ^1.14.12, flutter_test from sdk is incompatible with states_rebuilder >=2.0.0.

What does this mean? should I go back to collection 1.14.11?