Export native `fetch` from `@web-std/fetch`

[Gozala]

As per #51 it would be help to use fetch for loading assets in tests across node/browser. Which in turn would be straight forward if we @web-std/fetch could be used in test without having to use alternative code paths across node and web.

[jacob-ebey]

This project, the remix for, and even node-fetch are not as fully spec compliant as undici and I see issues arising in exiting codebases if this isn't a major. Specifically around locking of body streams / cloning behavior.