[Shell] statically expose the current shell

[StephaneDelcroix]

Summary

Getting a reference to the current shell is tedious and repetitive. As the shell has to be the App.MainPage, we could expose a static version of GoToAsync() that tries to get the shell from there. If the shell is not found, an exception will be thrown.

If the user want to navigate on a shell that's not currently App.MainPage (multiple shell app, anyone ?) he can still keep a reference to that shell and use the non-static GoToAsync

this is required for #4594

API Changes

public Task static Shell.GoToAsync(ShellNavigationState state, bool animate = true);

Intended Use Case

replace this usage

var shell = Application.Current.MainPage as Shell;
await shell.GoToAsync("app:///s/about?welcome=you");

by

await Shell.GoToAsync("app:///s/about?welcome=you");

[StephaneDelcroix]

of course, the proposed api is invalid

what about

Shell.CurrentShell.GoToAsync(...)