Add utility function to classify exceptions as failing the workflow vs failing the task

[dandavison]

See the waiting_for_handlers and waiting_for_handlers_and_compensation Python samples added in temporalio/samples-python#144, which feature a utility function like this:

def is_workflow_exit_exception(e: BaseException) -> bool:
    """
    True if the exception is of a type that will cause the workflow to exit.

    This is as opposed to exceptions that cause a workflow task failure, which
    are retried automatically by Temporal.
    """
    # 👉 If you have set additional failure_exception_types you should also
    # check for these here.
    return isinstance(e, (asyncio.CancelledError, exceptions.FailureError))