transform Stream

Make transform function behave differently when the input is Stream
```ts
rewrite.isStreaming // false
rewrite.transform(response.body as ReadableStream)
rewrite.isStreaming // true
rewrite.peek() // some string
await sleep(600 * ms)
rewrite.peek() // some string + additional string
rewrite.flush() // end stream
rewrite.isStreaming // false
rewrite.peek() // empty string
```
it will flush and immediately pass the output when calling `.then`
```ts
rewrite.isStreaming // false
await rewrite.transform(response.body as ReadableStream) // some string
rewrite.isStreaming // false
rewrite.peek() // empty string
```