MacOS new UNUserNotification API

[trungnt2910]

Currently being blocked by a weird bug, as described here.

[trungnt2910]

This is due to the sample application being unsigned, as documented here. Some documentation should be added for ToastNotification.Uno though.

[trungnt2910]

The new implementation should also be enabled by switching this variable to true:

ToastNotification.Uno/ToastNotification/ToastNotification.macOS.cs

Line 26 in 6c5fe1f

private static bool? _legacyBehavior = true;