feat: add ProcessPromise .verbose() for symmetry with.quiet()

[antongolub]

relates #710

const p = $`echo foo`
p.quiet()        // enable silent mode
p.quiet(false)   // then disable
p.verbose()      // enable verbose/debug mode
p.verbose(false) // and turn it off

await p

• Tests pass

[antonmedv]

Let's also improve docs: https://google.github.io/zx/configuration#verbose

Let's specify the difference between quiet & verbose? Then to use one or another?