Expose public API + make available safely

[nikku]

Is your feature request related to a problem? Please describe

Token simulation is a huge extension. Because of that it has a certain set of issues:

• Large "service footprint": It provides a massive number of services, non of which are namespaced. As a result it is easy to collide with existing (bpmn-js) services declared
• No public API: It does not provide any reasonable public API (no docs on usage either). For proper consumption we shall add API that allows external components to interface with it. https://github.com/bpmn-io/bpmn-js-token-simulation/pull/112 is a start, however it only does a poor mans job.

Describe the solution you'd like

• [ ] Investigate if auxiliary token-simulation services can be hidden from sight (using didi __exports__ or proper namespacing
• [ ] Investigate clear boundary / public API

Describe alternatives you've considered

Keep what exists, even if it is a little messy.

Additional context

Using didi __exports__ is properly working since since https://github.com/nikku/didi/pull/13; we could consider to integrate it / use it with bpmn-js-token-simulation.