Add notifications method to Session

[dbdbdb]

The documentation state that there should be a notifications function
on the Session to get events from all subscribed channels:

https://tpolecat.github.io/skunk/tutorial/Channels.html#listening-to-a-channel

The function existed on the protocol level but was missing in
the Session.

[matthughes]

I know this is pretty old but has this been overcome by events @dbdbdb ? There is a listen method on Session that has the same type signature. How is that different than your proposed method?