Update library to comply with bpmn.io baseline

[nikku]

Proposed Changes

This adjusts the library structure to comply with the standard pattern of bpmn.io libraries.

What is out of scope for now:

• I did not touch the testing approach
• I did not configure linting
• There is a number of "how this is built" things to be discussed, for example:
  • Should dmn-moddle be a dependency, vs. does this library consume a moddle tree?
    • Consuming a moddle tree has distinct advantages, i.e. the outside frame can decide which meta-model to use ➡️ https://github.com/bpmn-io/bpmn-js-differ employs this, too. ➡️ Work on moddle tree rather than XML strings #2
  • Align in structure with https://github.com/bpmn-io/bpmn-js-differ: Due to the different purpose and feature set (this library supports deep content addressing / diffing) I did not align this; will likely be a breaking change for bpmn-js-differ.

Checklist

To ensure you provided everything we need to look at your PR:

• Brief textual description of the changes present
• Visual demo attached
• Steps to try out present, i.e. using the @bpmn-io/sr tool
• Related issue linked via Closes {LINK_TO_ISSUE} or Related to {LINK_TO_ISSUE}