Shell Routing through attributes on page (using source generators)

### Description

Today, we must register each page in the AppShell on startup such as:

```csharp
public AppShell()
{
InitializeComponent();

Routing.RegisterRoute(nameof(DetailsPage), typeof(DetailsPage));
}
```

However, it would be nicer to align with how ASP.NET Core routing works and make it an attribute on the page. Since this would require assembly scanning today, this should be implemented as source generators for optimal performance.

### (Public) API Changes

```csharp
[Route(nameof(DetailsPage))]
public partial class DetailsPage : ContentPage

{

}
```

This would generate code necessary to register the `DetailsPage` with the route of `DetailsPage`.

### Usage Scenarios

See above :)

### Backward Compatibility

Would be an added option and an upgrade from the existing mechanism.

### Difficulty

Medium

Start with the AppShell startup example and the existing Routing.RegisterRoute call shown in the issue. Then investigate the proposed Route attribute and source-generator integration, including how DetailsPage would be discovered. Done means the attribute-based registration works while the existing mechanism remains available.

Written by the indexing model from the issue text.