Improve typescript support

[jnraine-stripe]

Notify

r? @richardm-stripe

Summary

This improves three friction points when working with Typescript:

1. new StripeResource(stripe) now works. Prior to this change, attempting to construct StripeResource with its one required argument would fail a type check. Support for a constructor was added in Fix StripeResource.extend type #1245 and extended in Fix StripeResource constructor type #1249 (never merged). Thanks @rattrayalex!
2. StripeResource.extend now returns a class that is properly typed when instantiated. Prior to this change, you could extend StripeResource but your extensions wouldn't appear on the object after construction. Now, it does.
3. StripeResource.method returns a function that returns Response<object>. Prior to this change, extensions created with StripeResource.method would have a return type of object. Returning Response<object> makes it possible to call functions like lastResponse without casting the value.
4. StripeResource.method now accepts generic type that sets the response object's type. For example, if you're hitting a customer endpoint, you can do this: StripeResource.method<Customer>(...). When you use that extension, it will return Response<Customer>. This eliminates the need to cast when calling StripeResource extension methods.

See changes to typescriptTest.ts for these updates in action.

---

fixes #1249

[CLAassistant]

All committers have signed the CLA.

[richardm-stripe]

This looks good to me!