chore(ai): export AsyncIterableStream type from async-iterable-stream module (#7370)

## Background

I am writing decorator functions for streamObject, and the function
signature is using AsyncIterableStream, so I need to have it exported
too.

## Summary

export AsyncIterableStream type from async-iterable-stream module

Author: jonaslalin

.changeset/heavy-teachers-repeat.md

@@ -0,0 +1,5 @@
+---
+'ai': patch
+---
+
+chore(ai): export AsyncIterableStream type from async-iterable-stream module

packages/ai/src/util/index.ts

@@ -1,3 +1,4 @@
+export type { AsyncIterableStream } from './async-iterable-stream';
 export { cosineSimilarity } from './cosine-similarity';
 export { getTextFromDataUrl } from './data-url';
 export type { DeepPartial } from './deep-partial';