Add chapter 10 sample project

Chapter10/TemplateStudioSampleApp/Activation/ActivationHandler.cs

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+namespace TemplateStudioSampleApp.Activation;
+
+// Extend this class to implement new ActivationHandlers. See DefaultActivationHandler for an example.
+// https://github.com/microsoft/TemplateStudio/blob/main/docs/WinUI/activation.md
+public abstract class ActivationHandler<T> : IActivationHandler
+    where T : class
+{
+    // Override this method to add the logic for whether to handle the activation.
+    protected virtual bool CanHandleInternal(T args) => true;
+
+    // Override this method to add the logic for your activation handler.
+    protected abstract Task HandleInternalAsync(T args);
+
+    public bool CanHandle(object args) => args is T && CanHandleInternal((args as T)!);
+
+    public async Task HandleAsync(object args) => await HandleInternalAsync((args as T)!);
+}