Add extensionElements to process

@maxtru
your example works great, but not i my example :frowning:

Take a look at the example (bpmn) that I sent in the main post (when the Process in the Participant)

<?xml version="1.0" encoding="UTF-8"?>
<bpmn:definitions xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:bpmn="http://www.omg.org/spec/BPMN/20100524/MODEL" xmlns:bpmndi="http://www.omg.org/spec/BPMN/20100524/DI" xmlns:dc="http://www.omg.org/spec/DD/20100524/DC" id="Definitions_1npd7tv" targetNamespace="http://bpmn.io/schema/bpmn" exporter="bpmn-js (https://demo.bpmn.io)" exporterVersion="7.0.0">
    <bpmn:collaboration id="Collaboration_14y77iu">
    <bpmn:participant id="Participant_19fcdqe" processRef="Process_1" />
    </bpmn:collaboration>
    <bpmn:process id="Process_1" name="Process_name" isExecutable="true">
        <bpmn:startEvent id="StartEvent_09wqlnn" />
    </bpmn:process>
    <bpmndi:BPMNDiagram id="BPMNDiagram_1">
    <bpmndi:BPMNPlane id="BPMNPlane_1" bpmnElement="Collaboration_14y77iu">
        <bpmndi:BPMNShape id="Participant_19fcdqe_di" bpmnElement="Participant_19fcdqe" isHorizontal="true">
        <dc:Bounds x="106" y="61" width="600" height="250" />
        </bpmndi:BPMNShape>
        <bpmndi:BPMNShape id="_BPMNShape_StartEvent_2" bpmnElement="StartEvent_09wqlnn">
        <dc:Bounds x="156" y="81" width="36" height="36" />
        </bpmndi:BPMNShape>
    </bpmndi:BPMNPlane>
    </bpmndi:BPMNDiagram>
</bpmn:definitions>

when i run my bpmn in your codesandbox i get null or undefined ( in const process )