[Feature] IPageVisibilityEvents interface

[BurkusCat]

Is your feature request related to a problem? Please describe.
A new interface that can be added to viewmodels called IPageVisibilityEvents. These events are 1:1 mappings of the view's equivalent events.

Describe the solution you'd like
Ideally need a simple implementation that wouldn't require the user to do any more setup than adding the interface to their viewmodel (i.e. it would be good if it wasn't required to override ContentPage with a BurkusContentPage etc.).

Describe alternatives you've considered
These events can be manually mapped and passed to the viewmodel today. Or code can be triggered from code-behind.

Proposed APIs

• OnAppearing()
  • Is triggered by the Page's OnAppearing event.
• OnDisappearing()
  • Is triggered by the Page's OnDisappearing event.

**Additional context**
N/A