Type `moddle` parts of the library

[nikku]

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

Through type definitions we already greatly enhanced the development experience (integrating our editors and toolkits).

Unfortunately these definitions do not cover the bpmn-moddle (ref) of our tooling yet, as a result diagramElement.businessObject will generally be untyped (any).

Hence mistakes in usage could easily happen, undetected for our users.

Describe the solution you'd like

• bpmn-moddle (diagramElement.businessObject) is typed

Describe alternatives you've considered

None.

Additional context

Depends on bpmn-io/bpmn-moddle#116.