Add full XAML support for .NET MAUI compatibility

New features:
- MauiAppBuilderExtensions.UseOpenMauiLinux() for standard MAUI integration
- New template: openmaui-linux-xaml with full XAML support
- Standard MAUI XAML syntax (ContentPage, VerticalStackLayout, etc.)
- Resource dictionaries (Colors.xaml, Styles.xaml)
- Compiled XAML support via MauiXaml items

Templates now available:
- dotnet new openmaui-linux      (code-based UI)
- dotnet new openmaui-linux-xaml (XAML-based UI)

Usage:
    var builder = MauiApp.CreateBuilder();
    builder
        .UseMauiApp<App>()
        .UseOpenMauiLinux();  // Enable Linux with XAML

templates/openmaui-linux-xaml-app/Program.cs

@@ -0,0 +1,16 @@
+using OpenMaui.Platform.Linux;
+
+namespace OpenMauiXamlApp;
+
+public class Program
+{
+    public static void Main(string[] args)
+    {
+        // Create the MAUI app using standard MAUI bootstrapping
+        var app = MauiProgram.CreateMauiApp();
+
+        // Run with Linux platform
+        // This connects MAUI's virtual views to our Skia platform views
+        LinuxApplication.Run(app);
+    }
+}