[enhancement] add icon to portal tabs

[IbisLiven]

Since we can have files open in a window that aren't involved in a teletype portal, it would be nice if there was an icon added to all file tabs that are being shared with teletype — especially since teletype now adds new tabs when the host opens a new buffer. Maybe just put the same icon used to create/join portals beside the filename.

[nathansobo]

Thanks for the feedback @IbisLiven. I agree that some sort of additional visual distinction could be helpful to add.