EMSCRIPTEN_FETCH_STREAM_DATA is not implemented

[ifiddynine]

EMSCRIPTEN_FETCH_STREAM_DATA was disabled due to prior use of moz-chunked-arraybuffer which isn't supported anymore (plus, was Firefox only). It would be great to rewrite either the fetch backend to use fetch, or to write a special path in case streaming is requested. Some info + examples here: https://developer.mozilla.org/en-US/docs/Web/API/Streams_API/Using_readable_streams

This is a known issue given the assert here: https://github.com/emscripten-core/emscripten/blob/main/src/Fetch.js#L278. We're interested in receiving chunked data so figured it could be useful to have an issue so the task isn't lost.

[brendandahl]

This has been fixed in #25383.