chore: add avalonia DataValidationErrors demo.

src/Avalonia/HandyControlDemo_Avalonia/ViewModel/Common/InputElementDemoViewModel.cs

@@ -0,0 +1,19 @@
+using System.ComponentModel.DataAnnotations;
+using CommunityToolkit.Mvvm.ComponentModel;
+
+namespace HandyControlDemo.ViewModel;
+
+public partial class InputElementDemoViewModel : ObservableValidator
+{
+    [Required] [EmailAddress] [ObservableProperty] [NotifyDataErrorInfo]
+    private string? _email1;
+
+    [Required] [EmailAddress] [ObservableProperty] [NotifyDataErrorInfo]
+    private string? _email2;
+
+    [Required] [ObservableProperty] [NotifyDataErrorInfo]
+    private string? _text1;
+
+    [Required] [ObservableProperty] [NotifyDataErrorInfo]
+    private string? _text2;
+}