Track places where a Variable is used

[marstamm]

Is your feature request related to a problem? Please describe.

For a complete overview of Process variable flows, we also need to track where data is consumed. This allows us to also create Lint warnings, such as "Written and never used" or "undefined variable".

Describe the solution you'd like

Add an additional variable property that tracks tasks where the variable was used

Describe alternatives you've considered

Additional context

related to https://github.com/camunda/product-hub/issues/2011