feat: enhance support for multiline cmd args

[antongolub]

closes #539

• Tests pass

const p1 = (await $`echo foo
     bar
     "baz
      qux"
`).toString()
// 'foo bar baz\n      qux\n'

const p2 = (await $$`echo foo \
                     bar \
                     baz \
`).toString(),
// 'foo bar baz\n'