The builder needs a way to add rootcomponents instead of relying on OnStart

Currently users have to do this to add a root component:

```csharp
.OnStart((sender, host) => { host.RootComponents.Add(new CloseDemoWindowRootComponent()); })
```

it would be nice to have the ability to do it like the `SetStartingScreen` does:

```csharp
.AddRootComponent<CloseDemoWindowRootComponent>()
.AddRootComponent(new CloseDemoWindowRootComponent())
```