Support a C#-style ContinueAsNew invocation

Follow-up to: https://github.com/Azure/azure-functions-durable-python/issues/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

Start by reviewing issue #148 and the JavaScript handling of `continueAsNew`; the issue names no files or tests. Verify that both yielded and direct-call styles are supported, then update the Azure documentation to describe or encourage the consistent invocation style.

Written by the indexing model from the issue text.