feat: Add ChooseIndex, AdvancedChooseIndex, and MultiChooseIndex.

[ad8-adriant]

Hi,

This PR adds variants of Choose, AdvancedChoose and MultiChoose that return the index of the selected choice(s) rather than the string value.

I've encountered a number of situations where I wanted to prompt a user to select from an existing slice of arbitrary Go values, dynamically transformed into prompt options. That initial transformation is easy enough, but things get awkward once an option is selected as only the prompt text is returned, requiring a string search of the generated options to find the index of the selected value.

The new methods remove the search step by returning the index, allowing the final result to be retrieved directly, e.g.:

// Asks the user to select an element of vals, transformed into prompt options by fn.
func ChooseFrom[VS ~[]V, V any](ask string, vals VS, fn func(V) choose.Choice) (V, error) {
  opts := make([]choose.Choice, len(vals))
  for idx, val := range vals {
    opts[idx] = fn(val)
  }
  idx, err := prompt.New().Ask(ask).AdvancedChooseIndex(opts)
  if err != nil {
    var nop V
    return nop, err
  }
  return vals[idx], err
}

type MyThing struct {
  Name string
  A, B int
}

var ts = []MyThing{
  {"First", 1, 2},
  {"Second", 3, 4},
  {"Third", 5, 6},
  {"Fourth", 7, 8},
  {"Fifth", 9, 10},
}

func main() {
  val, err := ChooseFrom("Pick a thing.", ts, func(v MyThing) choose.Choice {
    return choose.Choice{
      Text: v.Name,
      Note: fmt.Sprintf("A = %d, B = %d", v.A, v.B),
    }
  })
  if err != nil {
    panic(err)
  }
  fmt.Println(val)
}