Implements `renderToString` in terms of a new `renderToStream`. The former is used for legacy documents that generate the body HTML as part of `getInitialProps`. The latter will be used directly in #27794 when streaming dynamic HTML.
Since we're exposing an actual streaming response for dynamic HTML (instead of buffering with `resultFromChunks`), we use `multiplexResult` to buffer and multiplex the underlying result to multiple subscribers.
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