feat: apply `promisifyStream` to `ProcessPromise` exposed streams

We've [previously added](https://github.com/google/zx/pull/954) stream extensions to make support mixed piping:
```ts
await $`cmd`.pipe(transform).pipe`cmd`
```

Let's also apply the same approach for `ProcessPromise` exposed streams:
```ts
const p1 = $`cmd`

const p2 = p.stdout.pipe`cmd1`
const p3 = p.stderr.pipe`cmd2`
```

See `util.ts#promisifyStream` for details