Make the help customiztaion "easier"

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 AsynchronousCommandLineAction(ParseResult parseResult, CancellationToken cancellationToken);

```

- add helper methods to the root command which returns the Help and `VersionOption`s