Add foundation

Toolkit.Foundation/ValueViewModel.cs

@@ -0,0 +1,21 @@
+using CommunityToolkit.Mvvm.ComponentModel;
+
+namespace Toolkit.Foundation;
+
+public partial class ValueViewModel<TValue>(IServiceProvider serviceProvider,
+    IServiceFactory serviceFactory,
+    IPublisher publisher,
+    ISubscriber subscriber,
+    IDisposer disposer) :
+    ObservableViewModel(serviceProvider, serviceFactory, publisher, subscriber, disposer)
+{
+    [ObservableProperty]
+    private TValue? value;
+
+    protected virtual void OnChanged(TValue? value)
+    {
+
+    }
+
+    partial void OnValueChanged(TValue? value) => OnChanged(value);
+}