Preserve working directory across restarts

[jmcphers]

This change allows the working directory to persist across restarts, so that after a restart you don't need to re-navigate.

Most of the work is in the supervisor; this is just the client bits. I would have preferred for this change to only exist in the supervisor, but it turned out that on Windows there is no way to examine another process's current working directory if it has been changed, so the supervisor actually has no visibility into the working directory without trying to sniff the UI comm traffic or something equally dreadful.

Since we already plumb the working directory into the front end in order to display it in the Console, the approach taken here is to have the front end tell the supervisor the directory in which to perform the restart, since it already knows it.

Addresses #3188.

Release Notes

New Features

• The current working directory is now preserved across R and Python restarts (Restarting R or Python loses current working directory #3188)

Bug Fixes

• N/A

QA Notes

This should be tested on Windows because (predictably) it is the platform on which we have to do the most gymnastics to get this to work.

[github-actions]

E2E Tests 🚀
This PR will run tests tagged with: @:critical

^{readme  valid tags}

[seeM]

This is nice! Works well locally and code changes look good