fix: mixin ProcessOutput data to promisified pipe value

[antongolub]

continues #949

Promisified pipe result exposes the closest left ProcessOutput data for backward compatibility

const file = tempfile()
const fileStream = fs.createWriteStream(file)
const p = $`echo "hello"`
  .pipe(getUpperCaseTransform())
  .pipe(fileStream)
const o = await p

assert.ok(p instanceof WriteStream)
assert.ok(o instanceof WriteStream)
assert.equal(o.stdout, 'hello\n')
assert.equal(o.exitCode, 0)
assert.equal((await fs.readFile(file)).toString(), 'HELLO\n')

• Tests pass