feat: introduce abort() method

[antongolub]

closes #527

const p1 = $`sleep 9999`
setTimeout(() => p.abort(), 100)

const ac = new AbortController()
const p2 = $({ ac })`sleep 9999`
setTimeout(() => ac.abort(), 100)

• Tests pass

[ftoh]

Why support ac instead of signal, for which JavaScript has methods to work with?

const p1 = $({ signal: AbortSignal.timeout(100) })`sleep 9999`
 
// and

const ac = new AbortController()
const p2 = $({ 
  signal: AbortSignal.any(AbortSignal.timeout(1000), ac.signal)
})`sleep 9999`

button.addEventListener('click', () => {
  ac.abort()
})

[antongolub]

Reasonable, I'll add signal shortcut too.

[antonmedv]

Let’s drop ac in flavor of signal?