feat(demo): add AppShellConfiguratorImpl

Needed for demos compatibility with Vaadin 25.

Author: paodb

src/test/java/com/flowingcode/vaadin/addons/demo/AppShellConfiguratorImpl.java

@@ -0,0 +1,9 @@
+package com.flowingcode.vaadin.addons.demo;
+
+import com.vaadin.flow.component.page.AppShellConfigurator;
+import com.vaadin.flow.theme.Theme;
+
+@Theme
+public class AppShellConfiguratorImpl implements AppShellConfigurator {
+
+}