If a IsModal property is introduced, it should apply to ALL states, including custom ones the user adds to the States collection. We expect the following properties if IsModal is true:

- The user should not be able to interact with background content (unless dismissed)

- In terms of accessibility, keyboard navigation should only navigate within the RadBottomSheet.BottomSheetContent (so not the background or RadBottomSheet.Content)

- Preferably a visual indicator like a scrim in the background to indicate to the user that the bottom sheet is acting like a modal, that when tapped, dismisses both the scrim and the bottom sheet.