Declare ViewModelBase as abstract

[ssg]

Is your feature request related to a problem? Please describe.

ViewModelBase in the template is a regular class, so, it doesn't explicitly convey that it should be the basic building block of other view models in the app. That might be confusing to the beginners.

Describe the solution you'd like

Declare ViewModelBase abstract.

Describe alternatives you've considered

No response

Additional context

No response