Add function to retrieve actual input values for command options by name

[fa0311]

• Thanks for your contribution! Please replace this text with a description of what this PR is changing or adding and why, list any relevant issues, and review the contribution guidelines below.

---

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.

Contribution guidelines:

• See our contributor guide for general expectations for PRs.
• Larger or significant changes should be discussed in an issue before creating a PR.
• Contributions to our repos should follow the Dart style guide and use dart format.
• Most changes should add an entry to the changelog and may need to rev the pubspec package version.
• Changes to packages require corresponding tests.

Note that many Dart repos have a weekly cadence for reviewing PRs - please allow for some latency before initial review feedback.

Motivation to add this.

import 'package:args/args.dart';

void main(List<String> args) {
  var parser = ArgParser();
  parser.addOption('size', abbr: 's', defaultsTo: '20');

  var results = parser.parse(args);

  var size = int.tryParse(results.option('size')!);

  if (size == null) {
    print('Could not parse the ${results.actual('size')} as an integer');
  } else {
    print('The size is $size');
  }
}

$ dart sample.dart --size aa
Could not parse the --size as an integer
$ dart sample.dart -s aa
Could not parse the -s as an integer