Add get_prior_action to Application

[CanReader]

Summary

• Add Application.get_prior_action(), the counterpart of get_next_action()
• Returns the action that ran in the last step, or None if nothing has run yet
• Uses the __PRIOR_STEP value that is already kept in state

Closes #502

Testing

• Added test_app_get_prior_action, it checks before any step, after each step and after reset_to_entrypoint()
• tests/core/test_application.py passes locally, pre-commit hooks pass