feat(collaboration): implement multiple users

[imsnif]

This PR finalizes the implementation of the multiple users feature.

This PR adds:

1. Handling of multiple users focused on different tabs in Screen, as well as a UI for such (multiple users focused on different panes was already merged in a previous PR behind a feature flag).
2. A CLI to indicate whether a session is mirrored (previous behaviour where all users share a cursor) or not (each user has their own cursor and can focus on different panes or tabs - this is the default behaviour from now on). options --mirror-session true
3. E2e tests for both mirrored mode and multiple cursor mode.