Use `TransformStream` and return SSE stream

[smaeda-ks]

Moving most parsing logic into TransformStream, which is a more appropriate interface for such stream chunk processing.

Some notes:

• we still need ReadableStream to properly handle fragmented SSE chunks from OpenAI
  • OpenAI -> (fragmented stream SSE chunks) -> ReadableStream -> (eventsource-parser parses and restores each SSE event out of received fragmented SSE chunks) -> TransformStream (we parse each SSE event data (JSON) passed from ReadableStream and transform as needed) -> stream response (SSE) -> Client (browser)
• Clients are now also reading SSE stream emitted by edge functions using eventsource-parser. This makes client-side chunk data parsing logic much easier to work with.

internal link: https://vercel.slack.com/archives/C0289CGVAR2/p1680094016236659

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