impement tmux for shared terminal

[jthunderbird]

Tmux can be used to share live terminals. Ideally, this solution brings the other user directly into an instance of code-server with terminal already up and connected to a central tmux session for n number of logins.

In limited testing, sudo was needed to start and attach to sessions; hopefully that can be fixed.

[jthunderbird]

This would basically replace the current live share mechanism for sharing access. May be or may not be useful - one way or the other should be implemented.