feat(js/flows): consolidated defineFlow and defineStreamingFlow

[pavelgj]

Deprecated defineStreamingFlow (not removed, still works). Streaming and non-streaming flows can be defined using defineFlow

    const streamingBananaFlow = ai.defineFlow(
      {
        name: 'banana',
        streamSchema: z.string(),
      },
      (input, sendChunk) => {
        for (let i = 0; i < input.length; i++) {
          sendChunk(input.charAt(i));
        }
        return input;
      }
    );

Also made it so that streaming callback (see sendChunk in the example above) is always defined. If flow is invoked in non-streaming mode a no-op callback is passed in.

Checklist (if applicable):

• Tested (manually, unit tested, etc.)