fix(button-group): adding new selectionMode sample

[georgianastasov]

Closes #3393

[kdinev]

I believe you need to add a setup for stackblitz, or this was in a different repository?

[teodosiah]

I think this is the same sample that is in the Button Group Multiple selection topic but modified, so it should be already setup.
However, @georgianastasov you need to import the IgxRadioModule in the ButtonGroupConfigGenerator.ts and also change the display name in the data-entries-routes-data.ts

[georgianastasov]

Yes this is the same sample that is in the Button Group Multiple Selection section but modified, as we updating the Multi Selection section and the sample to the Selection Mode section with sample showing the new selectionMode property with its new options single, singleRequired and multi.