feat(ai/rsc): Patchable string values

[shuding]

This makes sure that when sending a streamed string over createStreamableValue will be optimized so that only the diff gets sent over the wire. This will be a common use case for generated content.

When can also look into a special^* JSON diff-patch protocol for this API, so the shape will no longer matter.

*: We probably don't need fancy and expensive features like full text diff. Append-only is good enough for streaming cases.