Initial navigation when using region navigation can be simplified by setting IsDefault on Main route

[nickrandolph]

What would you like to be added:

Instead of calling Navigate from within ShellViewModel/Model, set the IsDefault property on the Main route.

Why is this needed:

Currently initial navigation is done using an async method from the constructor in ShellViewModel/Model. This isn't great because it's discarding the task and not a pattern that should be encouraged

For which Platform:

• [ ] iOS
• [ ] Android
• [ ] WebAssembly
• [ ] WebAssembly renders for Xamarin.Forms
• [ ] Windows
• [ ] Build tasks

Anything else we need to know?