Provide a way to see (and attach to?) running supervisor processes

[jmcphers]

The changes in #5899 made it possible to close Positron while leaving the supervisor running for a period of time (perhaps indefinitely!). This is useful for letting computations run in the background and returning to them later, or leaving a data set warm in memory.

However, these supervisor sessions can easily become orphaned, especially if they are set to run indefinitely. The only way to end one (again, if you haven't specified a timeout) is to open the workspace associated with it and restart its supervisor. It is also difficult to know what's still alive.

It would be helpful to provide:

• a way to see all the running supervisor processes on the host (maybe on the welcome screen?) along with which workspace they are bound to and which sessions are running
• a way to open the workspace associated with a particular supervisor so you can resume the sessions associated with it
• a way to end any unwanted/unneeded supervisors

(via suggestion from @sharon-wang in #5899)

[ZhimingYe]

This will be an amazing feature! In my personal opinion, this might also be a good position to reimagine the background jobs (#3173). They are similar in some points. 😄