Implement ElementRegistry#getGraphics

__Is your feature request related to a problem? Please describe__

Like we have in [`diagram.js`](https://github.com/bpmn-io/diagram-js/blob/develop/lib/core/ElementRegistry.js#L182), it would make sense to have a `getGraphics` functionality for the `ElementRegistry` in `table-js`.

From an APIs perspective, it would be nice to have this aligned in all core libraries. I humbled over this while implementing test cases inside `dmn-js-decision-table` and get the DOM node from an element.

__Describe the solution you'd like__

Implement `ElementRegistry#getGraphics` to return a SVGElement oder a DOMNode.

__Describe alternatives you've considered__

Do nothing because it's not feasible.

__Additional context__

[`diagram.js` Implementation](https://github.com/bpmn-io/diagram-js/blob/develop/lib/core/ElementRegistry.js#L182)

The requested entry point is ElementRegistry#getGraphics; start by comparing it with the linked diagram.js implementation. Done means the table-js ElementRegistry exposes getGraphics and returns an SVGElement or DOMNode for an element.

Written by the indexing model from the issue text.