This allows the registration of such buffers in the project, which is necessary
to correctly support `::save_buffer_as` and opens the door to sharing untitled
buffers with guests in the future.
Note that, for now, this disallows guests to create untitled buffers in the
current window and will create a new window instead. This is because we don't
yet have a global way of allocating a buffer's remote id (nor a way of saving
such buffers in the host's worktree) and we instead rely on the local model ID,
which could clash with the host's buffer IDs.
I think we should revisit this once guests can share their untitled buffers
with the host and other remote peers, as well as once we start keying
operations by entry id.
29cad65ce0