Remove bpmn references from dict.py and json.py

[jbirddog]

Diff is on the larger side but I tried to change as little logic as possible. The main changes here are splitting DictionarySerializer and JSONSerializer in "core" and moving the bpmn specific parts to BPMNDictionarySerializer in bpmn/serializer/dict.pyandBPMNJSONSerializerinbpmn/serializer/json.py. The bpmn specific serializers are subclasses of the core serializers and only override when needed. I believe the split is correct but if anything looks off please let me know. For JSONSerializer` I also moved top level functions to class methods to make things easier on the subclass.

If I grepped correctly this combined with the other bpmn import related prs purge core of all bpmn/dmn/camunda references.

[sonarcloud]

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