Provide type definitions for improved editor assistance

[nikku]

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

As I use this library I do not get the assistance that I'd like to get in my editor tooling.

Describe the solution you'd like

• As I work with the library as part of my project I get type that guide me on how to use the tool.
• The type definition accounts for various use-cases, including the standard extension use-case that we provide.

Describe alternatives you've considered

None.

Additional context

This has been brought up in a community conversation. Initial ground work happened in other parts of bpmn.io already.