Fix DRD not working with diagram-js@4

[nikku]

Describe the Problem

diagram-js@4 added the ability to swap connection source and target in a way that is incompatible with dmn-js-drd@6. Quick attempts to rewrite the DMN logic failed thus this issue.

Expected Behavior

• dmn-js-drd is fully functional and working on top of diagram-js@4+
• If it helps us, the source / target swapping concern is better supported as a core functionality in diagram-js (cf. Add ability to swap source and target on connect and reconnect diagram-js#386)

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Related to bpmn-io/diagram-js#386