Add ExtendedSwitch Control - Advanced Toggle Switch (#17)

Introduces a new ExtendedSwitch control with rich customization options and integrates it into the TestApp as a demo page.

Adds the ExtendedSwitch control class with 25+ DependencyProperties, programmable template, and smooth animations.
Creates a TestExtendedSwitch demo page (XAML + code-behind) in the TestApp, registers it in the navigation registry, and includes it in the project file.
Updates the TestApp’s project and registry to expose the new demo page.

Author: vickyqu115