MVVM Toolkit improvements

[Sergio0694]

Suggested new feature or improvement

As discussed at the Dev Home booth at Build, there's a few log hanging fruits with the MVVM Toolkit here:

• [ ] Updating to 8.2
• [ ] Switching types from ObservableRecipient to ObservableObject
• [ ] Switching some obvious properties/commands to source generators

For (1), the main advantages of this updates are:

• The source generators are massively faster and more efficient
• There's new analyzers to help detect bugs
• There's new features that can be used in the future

The full changelog for the two releases are here:

• Announcing .NET Community Toolkit 8.1
• Announcing .NET Community Toolkit 8.2

For (2) and (3), mostly:

• If messaging features aren't used (which is the case here), using ObservableRecipient is just wasteful.
• For those properties/commands, using the source generators just makes the code less verbose and error prone.