Add variable explorer icon to toolbar

[claudiaregio]

trafficstars

Since we are shipping the variable explorer, let's add the icon to match that of Jupyter to maintain consistency across experiences + increase discoverability for our users. (+telemetry that shows us how often it's being opened through the toolbar icon)

[jonsequitur]

Can you show an example of where this button would go?

[claudiaregio]

Python experience has it just to the left of Table of Contents icon