Add a rule `no-overlapping-elements`

Is your feature request related to a problem? Please describe

Overlapping elements are visually hard to comprehend (ie. you might miss a label). Ideally elements don't overlap, apart from some exceptions:

Boundary events overlap their host
Child elements overlap / are on top of their parent (e.g., elements within a subProcess)

Describe the solution you'd like

Introduce a rule no-overlapping-elements, which would give a warning, if two elements overlap (except for exceptions, see above).

Example of where the rule should give a warning: Screenshot_20220216_163526

Describe alternatives you've considered

n/a

Additional context

Maybe this could be merged with rule to be built as part of https://github.com/bpmn-io/bpmnlint/issues/23

bpmn-io / bpmnlint