Start supervisor concurrently with Positron server; address some state issues

[jmcphers]

This change addresses some of the edge cases around the relationship between Positron Workbench sessions, browser tab sessions, extension host sessions, supervisor sessions, and R/Python sessions.

The primary change is the way the supervisor is started in Workbench. Formerly, it worked the same way it did on Desktop: every workspace started its own supervisor. Now:

• The supervisor is started at the same time as the shared Node process
• All of the workspaces in the Positron Workbench session share that supervisor

This improves performance, because the supervisor is already warm by the time we're ready to start a session. It also fixes some state issues that arise from storing the supervisor connection information in workspace state, which is too durable; on Workbench, the connection information now comes from a session-scoped file path.

The remainder of the change is around handling multiple tabs connecting to the same workspace. When this happens, you will now see a dialog explaining what happened, along with a message in the Console showing that your sessions have been transferred to the new window.

Positron should otherwise keep running fine in the old window if you have any non-data-related work to do, such as finishing up with script edits or Terminal tasks.

You might be wondering what happens if you start a new session in a tab after its existing sessions get moved to another tab. The system is designed to handle this; you can perform this move to get two tabs with different sets of sessions going against one workspace if you really want/need to. Opening a new tab (or reloading an existing one) will cause it to adopt all the sessions from any other tabs/windows, by design, so that you can't orphan any sessions.

Addresses #5374
Addresses #6174
Addresses #6799
Addresses #7157
Partially addresses #7078 (workbench only)

Release Notes

New Features

• N/A

Bug Fixes

• Fix 404 error when switching back to a previously open project in the same Positron window (Desktop: Switching back to a previously open project in the same window results in HTTP errors #6799)

QA Notes

This is a high-risk change because it affects how we start and reconnect to sessions, which involves a lot of different pieces (see list of "sessions" above) plus a lot of different state stored in a lot of different places.

Things that deserve extra attention:

• moving sessions to and from different tabs
• switching workspaces and windows
• reloading desktop & browser tabs
• switching browsers (note that console contents will not be preserved when you do this outside Workbench, since it is stored in the browser in that configuraiton)
• reloading/restarting kernels

Also note that in web mode, we can no longer respect the supervisor log level setting or the idle shutdown hours setting -- there is no (reasonable) way for us to read these settings at the time the supervisor is started. The latter setting doesn't make sense in web mode anyway; if we need to adjust the log level in web mode we will need to build another way to do it.

Test tags: @:sessions

[github-actions]

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

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