Notebook variables pane sync

[nstrayer]

Addresses #2452

This PR adds the ability for the variables pane to "follow" the user around as they work across different sessions (particularly useful for notebooks that have their own sessions.)

It works by automatically switching the variables pane in the following circumstances

• If the active editor tab is changed
  • If changed to a notebook, will switch variables to that notesbook session's variables
  • If changed to a non-notebook file such as a script, will switch the variables to the last-active console session.
• If code is run in the console the session backing the console's variables will be focused.

variables-follow-mode.mov

All of this behavior can be turned off by setting the positron.variables.followMode setting to false. (src/vs/workbench/services/positronVariables/common/positronVariables.contribution.ts)

Annoyances

• Currently there's no real way to correlate a script like a .R or .py script with a session. In this case we go back to the most recent console session, but that could be confusing if you clicked onto an R session and your most recent console was a python session. I left this behavior as is because it's what currently happens and it feels like the upcoming multi-session work will formalize behavior around this.

Code Refactoring:

• Renamed the method setActivePositronVariablesInstance to _setActivePositronVariablesInstance for consistency with other private methods. (src/vs/workbench/services/positronVariables/common/positronVariablesService.ts)

QA notes:

Should be able to fiddle between different notebooks and consoles and the variables pane should go to where you expect it to.

[seeM]

Code changes LGTM and confirmed the expected behavior locally. A few thoughts below:

Should variables also select the correct runtime on Positron startup? For example, I had a notebook opened from a previous session, and when I started Positron it wasn't selected. We might just need to check the active editor etc when we also register the event listeners.

Also, should variables switch to the notebook runtime when a notebook cell is executed?

[seeM]

LGTM, thanks for laying the groundwork for unit testing the variables service!