feat(notification): replace MessageBox with toast notifications

[CalvinAllen]

Summary

• Add Windows toast notifications as an alternative to MessageBox for breakpoint hit notifications
• Create Options page accessible via Tools > Options > Breakpoint Notifier > General
• Three notification styles: MessageBox, Toast (default), or Both

Changes

• Add Microsoft.Toolkit.Uwp.Notifications NuGet package
• Create Options/GeneralOptions.cs with NotificationStyle enum and DialogPage
• Update BreakpointNotifierPackage.cs to register the Options page
• Update DebuggerEvents.cs with toast notification logic

Test plan

• Build the solution
• Launch experimental VS instance (F5)
• Open Tools > Options > Breakpoint Notifier > General
• Test "MessageBox" setting - verify MessageBox appears on breakpoint
• Test "Toast" setting - verify toast notification appears
• Test "Both" setting - verify both appear

Closes #4