Add ExtendedButton Control - Flexible Button Customization via Rich DependencyProperties (#16)

Adds an ExtendedButton control with rich, XAML-driven customization and updates the test app to include a demonstration page.

Registers and compiles a new TestExtendedButtonControl page in the sample app.
Implements the ExtendedButton control with 25+ DependencyProperties for declarative styling.
Provides a comprehensive demo page showcasing various button styles, states, and layouts.

Author: vickyqu115