[package:args] Add function to retrieve actual input values for command options by name.

[fa0311]

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Add function to retrieve actual input values for command options by name.
Continuation of dart-lang/args#284

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

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PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
args	Breaking	2.6.1-wip	2.6.1-wip	2.6.1-wip	✔️

Changelog Entry ✔️

Package	Changed Files

Changes to files need to be accounted for in their respective changelogs.

Coverage ✔️

File	Coverage
pkgs/args/lib/src/arg_results.dart	💚 88 %
pkgs/args/lib/src/parser.dart	💚 99 %

This check for test coverage is informational (issues shown here will not fail the PR).

API leaks ✔️

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Package	Leaked API symbols

License Headers ✔️

// Copyright (c) 2024, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

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