Make the help customiztaion "easier"

[zgabi]

The documentation (https://learn.microsoft.com/en-us/dotnet/standard/commandline/how-to-customize-help) shows that when the user wants to customize the help he needs to write a lot of unnecessary code. Create a class which in inherited from SynchronousCommandLineAction / AsynchronousCommandLineAction, find the HelpOption, etc...

I'd suggest you to simplify this.

For example:

• the AnonymousSynchronousCommandLineAction / AnonymousAsynchronousCommandLineAction classes (which are used in the SetAction methods) could be public. So the user can write this:

                defaultHelpOption.Action = new AnonymousSynchronousCommandLineAction(parseResult =>
                {
                    Console.WriteLine("HEADER");
                    return oldAction.Invoke(parseResult);
                });

• or replace the XXCommandLineAction classes with just simple delegates:

    public delegate int SynchronousCommandLineAction(ParseResult parseResult);
    public delegate Task<int> AsynchronousCommandLineAction(ParseResult parseResult, CancellationToken cancellationToken);

• add helper methods to the root command which returns the Help and VersionOptions

[jonsequitur]

Related: #2588