feat: add form infra.

src/Ursa/Controls/Form/Form.cs

@@ -0,0 +1,25 @@
+using Avalonia;
+using Avalonia.Collections;
+using Avalonia.Controls;
+using Avalonia.Markup.Xaml.Templates;
+using Avalonia.Metadata;
+
+namespace Ursa.Controls;
+
+public class Form: ItemsControl
+{
+
+    public static readonly StyledProperty<AvaloniaList<FormItem>> ItemsProperty = AvaloniaProperty.Register<Form, AvaloniaList<FormItem>>(
+        "Items");
+    
+    public AvaloniaList<FormItem> Items
+    {
+        get => GetValue(ItemsProperty);
+        set => SetValue(ItemsProperty, value);
+    }
+
+    public void AddChild(object child)
+    {
+        throw new NotImplementedException();
+    }
+}