feat: Prepend session name to terminal title

[injust]

Before trying shpool, I used screen to keep long-running commands around. I used screen's hardstatus to prepend the session name to the terminal title, so I could easily jump to the correct tab. I use a basic hardstatus string like "[%S] %h" (%Sis the session name and%h` is the original title).

It would be nice if shpool could do something similar, which would be in line with the existing prompt_prefix.

I tried to do this for shpool using fish_title, but it doesn't work well when re-attaching to a session with a long-running command. Since fish only updates the title when you hit a prompt, the title gets stuck on the last command you ran (i.e. shpool attach foo). #288 would make this possible.