Add support for Tizen

Code for Tizen has already been added in the [`unsupported-platforms`](https://github.com/AzureAms/ToastNotification.Uno/tree/unsupported-platforms/ToastNotification.Tizen) branch.

However, this code has not been tested. We will need to:

- Set up the [Tizen.NET](https://docs.tizen.org/application/vstools/install/) SDK.
- Build the sample app [here](https://github.com/AzureAms/ToastNotification.Uno/tree/master/ToastNotificationDemo/ToastNotificationDemo.Skia.Tizen), and potentially update any Uno.UI dependencies to fix bugs.
- Copy the Tizen platform code to your local branch and open a pull request to `master`. Please report any issues or success, with screenshots in the pull request.