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

[fa0311]

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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

[mosuem]

Closing as the dart-lang/args repository is merged into the dart-lang/core monorepo. Please re-open this PR there!