Work on moddle tree rather than XML strings

[nikku]

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

At its core moddle offers us the ability to plug in additional meta-data; this allows us to view contents of a (BPMN, DMN) document differently. Currently this library works of the XML string, hence it cannot support custom moddle trees and hardcodes the schema to use for parsing the XML.

I think it would be better to align this behavior with bpmn-js-differ and consume a moddle tree. This would also vastly simplify the API, which currently has to be asynchronous due to parsing the XML.

Describe the solution you'd like

• Work on moddle trees, not XML strings

Describe alternatives you've considered

None.

Additional context

Found in the context of #1.