Be able to group by variable name or element

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

> This is based on the [upcoming layout](https://github.com/camunda/product-hub/issues/3331) with variables in side bar instead of bottom bar

There are use cases in which users want to see a list of all variable names and in which users want to see a list of all elements that use variables.

### Describe the solution you'd like

Users should be able to group variables by their name or the elements in which they are used.

A sketch of what this could look like:


### Describe alternatives you've considered

Decide for one way of grouping and don't let users choose.

### Additional context

Related to https://github.com/camunda/camunda-modeler/issues/5623
Related to https://github.com/camunda/product-hub/issues/3331

Start by reviewing the requested grouping behavior and the linked upcoming layout in product-hub issue 3331, then read the related camunda-modeler issue 5623 for surrounding context. Done means users can choose to group variables by variable name or by the elements that use them, with the result matching the intended sketch.

Written by the indexing model from the issue text.