Rework to forward by default, and update readme

[jkarneges]

This reworks the kit to forward through Fanout by default (rather than forwarding without Fanout by default), and explicitly marks the app-handled endpoints as test endpoints (/stream/ -> /test/). Also, the readme has been corrected in a few places (notably the API call example was using the pre-api.fastly.com style).

The goal of this change is to:

1. Unify the two Fanout starter kits. This one kit will be suitable for either forwarding or as a starting point for edge apps, and the fanout-forward kit can be deprecated.
2. Enable using Fanout without having to understand Compute. The kit does not require modification for production use. Users can set and forget.

The test endpoints only work for domains ending in .edgecompute.app. This is similar to the original Fanout.io behavior, where test endpoints are not enabled on custom domains.

[harmony7]

I like it!