Support a C#-style ContinueAsNew invocation

[davidmrdavid]

Follow-up to: Azure/azure-functions-durable-python#148

In Python we handle both the case where continueAsNew is yield'ed and the case where it is simply called. In both cases, it works as expected.

We needed to handle both cases because JS already had the precedent of requiring a yield for continueAsNew, which departs from the C# style. We should follow-up on this difference and make JS support both calling styles as well.

Follow-up Items:

• To update the azure docs to describe this behavior, or at least to encourage the C# invocation style, for consistency