Provide a better way to hook custom help

Having to define `internal sealed class CustomHelpAction(HelpAction action) : SynchronousCommandLineAction` and then loop through Options on the `rootCommand` looking for `is HelpOption` is a lot of ceremony.

Could we have something like

```c#
var rootCommand = new RootCommand("RootCommand")
{
myArgument,
myOption1,
myOption2,
HelpAction => () { Console.Writeline("Easy help!") }
};
```

or

```c#
rootCommand.SetHelp((cancellationToken) =>
{
Console.Writeline("Easy help!")
}
```