Streaming support in `TextBlock`

While the SDK supports streaming for various message blocks (e.g., tool usage/result, system message, result message, text message etc.), `TextBlock` content is only delivered once it's fully available. This delays rendering for long messages, even if TTFT is low. Supporting partial streaming of `TextBlock` content would let applications render text incrementally, improving user experience a lot.

In current streaming guide, we receive entire block at once:

https://github.com/anthropics/claude-code-sdk-python/blob/3010aaf092186a24dae240d78f0ee7d2e0d5d62a/examples/streaming_mode_ipython.py#L26