Run line by line stop button is different from cell stop

[miguelsolorio]

Testing microsoft/vscode#129467

The one on the left of the cell is using codicon-notebook-stop and the one in the toolbar is codicon-debug-stop. I'd say we use the same codicon-notebook-stop icon.

[miguelsolorio]

Regarding this note:

There are two stop buttons visible (the normal cancel button and the RBL stop button in the cell toolbar) but these will be merged

If run line by line goes into the consolidated run buttons, then it makes sense to not show anything in the cell toolbar. If it's visible in the cell toolbar only, then i think hiding the one to the left makes sense. I still think we should only use the codicon-notebook-stop icon