Add durable methods to `app` object in v4

[ejizba]

Specifically this issue is about adding the app.durable methods that will provide nice intellisense about the trigger options when registering a function.

Support in the durable package (the part that makes durable actually run locally) is tracked here: Azure/azure-functions-durable-js#384

[ejizba]

There have been discussions offline about where to put the app.durable methods (in this package or the durable package). I had assumed they would be in this package, which is why the original issue was created here. I think I mostly assumed that because all the other important bindings will be here, but durable is pretty unique with its own sdk so 🤷‍♂️. Curious to hear the reasons for putting those methods in the durable package

Fwiw, I don't think it'll be that difficult either way. We would not drastically change anything in this module. Rather, the durable package would just provide wrappers around the app.generic method if they want to provide durable methods from their own package

I do think it increases discoverability if those methods are in this package as we focused on intellisense a lot in the new model and they would need to be in this package to show up when a user types app. by default.

cc @davidmrdavid

[ejizba]

Moved to durable repo and merged in Azure/azure-functions-durable-js#399