[IDE] worksheet run output is not preserved when switching tabs

- Open a worksheet in the IDE and run it, observe its output on the side
- Open another file in the IDE in a new tab
- Switch back to the worksheet tab
- Observe that the output is now gone

As far as I can tell, the proper way to do this is to store the output ourselves and apply the decorations when the current tab is a worksheet (using the onDidChangeActiveTextEditor callback).