enable experimental response streaming

[Rich-Harris]

This enables streaming from serverless functions on Vercel (edge functions already support streaming)

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[Rich-Harris]

(Further context: despite the experimental label this is ready for widespread use, and in fact is used for Next's new route handlers. According to benchmarks it's indistinguishable in performance from the non-streaming version, so the recommendation from the relevant people at Vercel is that we should enable it by default — no need to add an option for it.)