[Extension API] Allow determining whether the current user is following another participant

[lostintangent]

In order to support extensions that contribute unique and collaborative user interfaces (e.g. a whiteboard, integrated browser previews), they need to be able to detect if a participant is following someone, so that they can focus the appropriate UI in response to navigations by the participant being followed (e.g. launching a browser preview window).

Additionally, since the “Participants” list in the Live Share view displays each user’s active location (e.g. a file position), extensions should be able to update this location to refer to non-file-based documents to reflect each user’s actual focus (e.g. viewing a whiteboard, browsing a web page).

[fubaduba]

We’re not able to prioritize this issue over the other higher-impact issues we receive every week, based on the votes and comments from others in the community and our understanding of the issue. However, rest assured that we love your input. If you feel it deserves to stay open, then clarify your use case and contact us to let us know how severe it’s for you.