Add elementTemplates.createElement for Camunda Cloud

[pinussilvestrus]

This adds a basic createElement API to create elements out of a template descriptor.

const elementTemplates = modeler.get('elementTemplates');
const create = modeler.get('create');

const element = elementTemplates.createElement(template);
create.start(canvasEvent({ x: 250, y: 300 }), element);

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Closes #582

• Some context regarding the element factory approach: https://miro.com/app/board/uXjVOIOoGto=/?moveToWidget=3458764520184901766&cot=14
• Not in scope: Camunda platform bindings, cf. Add elementTemplates.createElement API for Camunda Platform  #607

[pinussilvestrus]

For the reviewer: if more context about these adjustments is needed, feel free to approach me 👍

[nikku]

Looks good.