Warn on `finally: cancel_scope.cancel()`

Like async102 and async120, this is to address the concerns discussed in https://github.com/python-trio/trio/issues/455. (also: should the rules docs link there?)

with CancelScope() as scope:
    try:
        ...  # whatever sync and async code here
    finally:
        scope.cancel()
        # You'd hope this just stops any ongoing work, but it swallows exceptions too!

In most cases the solution is simply to scope.cancel() without the try/finally block - structured concurrency means we reliably clean up on exceptions anyway. For some, the cancel scope is entirely unnecessary, and you could call e.g. recv_chan.close() instead of cancelling.

Start by reading the existing async102 and async120 rules and the concerns in Trio issue 455. Identify the rule implementation, tests, and documentation locations from those rules. Done means the linter warns for the shown finally/cancel pattern and the rules documentation links to the relevant discussion if appropriate.