Simple ICommandHandler that injects the ParseResult

Idea for an alternative to SetHandler() that is used in the example here.

class ParseResultHandler : ICommandHandler
{
    public ParseResultHandler(Action<ParseResult> handle) { ... }
    
    ...
}

static class Program {
    static int Main(string[] args)
    {
        var inputFileArg = new Argument<FileInfo>("input", "Path to the input file");
        var exampleOpt = new Option<int?>(new[] { "-n", "--number" }, "Example option");

        var rootCommand = new RootCommand(Description)
        {
            inputFileArg,
            exampleOpt,
        };

        rootCommand.Handler = new ParseResultHandler((result) => {
            var input_path = result.GetValueForArgument(inputFileArg);
            var option = result.GetValueForOption(exampleOpt);
            Console.WriteLine("Input path: {0}", input_path?.FullName ?? "None");
            Console.WriteLine("Option: {0}", option?.ToString() ?? "None");
        });

        return rootCommand.Invoke(args);
    }
}

I like this approach because it is easier for me to wrap my head around how it is working. Whereas I don't really understand what SetHandler<T1, T2, ...>() is doing except that it creates a handle function for AnonymousCommandHandler that eventually calls context.ParseResult.GetValueForArgument() and context.ParseResult.GetValueForOption() anyways.

Inizia da docs/Your-first-app-with-System-CommandLine.md e dalle API SetHandler e ICommandHandler esistenti menzionate nell'issue. Confronta il modo in cui SetHandler raggiunge ParseResult e determina l'ambito previsto e la forma dell'API per un ParseResultHandler; il lavoro sarebbe completo quando fossero disponibili un design documentato e accettato e una relativa implementazione o un esempio.