feat: make `ProcessPromise` iterable via async for

[antongolub]

Imagine:

const p = $`cmd`
for await (const chunk of p) {
  // ...
}

const o = await $`cmd`
for (const chunk of o) {
  // ...
}

Keep in mind, It is important to make iterator available at any stage of the process. See how the pipe() API handles this requirement.