Basic infrastructure is in place and now I will focus on fixing the errors.

I've fixed the types, but I believe we should also cross-test this with bpmn-js-properties-panel or have an integration-test app similar to the one in form-js before we merge this.

I created a branch which implements the type linting in bpmn-js-properties-panel but it's far from being finished. Check out the link: https://github.com/bpmn-io/bpmn-js-properties-panel/tree/add-type-linting

Although the type linting results are all green, the hundreds of errors within the integration test branch linked above makes me believe we don't sufficiently test the declarations. Also, it's not that easy to handle the broken JSDoc typings we have in bpmn-js-properties-panel. This means I don't expect that we quickly adopt type linting in that project and thus cannot verify the typings here.

Consequently, I am closing this PR. I believe we should start with limited, public API-only declarations written by hand. Regarding the code in this PR, there are some fixes which we can use (e.g. spellCheck fix) in a separate contribution.