feat: support pipe splitting (like tee) and delayed piping

[antongolub]

Status:
stage 0 proposal

Demo

const result = $`echo 1; sleep 1; echo 2; sleep 1; echo 3`
const piped1 = result.pipe`cat`
let piped2

setTimeout(() => {
  piped2 = result.pipe`cat`
}, 1500)

await piped1
assert.equal((await piped1).toString(), '1\n2\n3\n')
assert.equal((await piped2).toString(), '1\n2\n3\n')

Bound Сhanges

• stdio: ['inherit', 'pipe', 'pipe'] → stdio: pipe
• p.halt() → $({halt: true})

nodejs/help#2707

• Tests pass
• Appropriate changes to README are included in PR