perf(web): fast path for returning Response

[pi0]

Normally when we return a value for h3, it tries to detect the content type and prepare a response/response-init. When users directly return a Response object (and have not preset any other headers or status/statusCode), we can go through a fast path optimization making handling %3 to %6 faster.

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But there is another part in the story. When h3 is combined with other web framework handlers that always return a Response object, we were blindly creating another Response object because it was working based on merging.

By smarty reusing the same Response object, we can speed up to 2x (Bun) (%25 on Node) in performances.

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This change also has the benefit that onBeforeResponse has access to the final prepared Response object instead of prepared ResponseInit