feat(ai/rsc): readStreamableValue

[shuding]

This PR removes the useStreamableValue API and implements readStreamableValue instead.

readStreamableValue takes a streamable value created via the createStreamableValue().value API, and returns an async iterator.

// Inside your AI action:
async function action() {
  'use server'
  const streamable = createStreamableValue();
  streamable.update(1);
  streamable.update(2);
  streamable.done(3);
  // ...
  return streamable.value;
}

And to read the value:

const streamableValue = await action()
for await (const v of readStreamableValue(streamableValue)) {
  console.log(v)
}

This logs out 1, 2, 3 on console.

This API is simpler and more imperative. It gives back retries, error handling and state tracking to the userland.