Add generic host builder pattern for System.CommandLine.Hosting

Align `System.CommadLine.Hosting` model with the way ASP.NET Core uses _Generic Host_.

It should be possible to configure CommandLineBuilder like this:
```csharp
public static IHostBuilder CreateHostBuilder(string[] args) =>
CommandLineHost.CreateDefaultBuilder(args)
.ConfigureAppConfiguration((hostingContext, config) =>
{
})
.ConfigureCommandLineDefaults(cmdHost =>
{
cmdHost.ConfigureServices(services =>
{
})
})
.ConfigureCommandLineBuilder(cmdBuilder => // CommandLineBuilder
{
cmdBuilder.UseReflectionAppModel(); // e.g. https://github.com/KathleenDollard/command-line-api-starfruit
});
});
```