[FEAT] Let update the style of a BpmnElement returned by the APIs

[csouchet]

This follows implementation of #1030

In a second iteration, we propose to change the style by functions via the BpmnElement object retrieved using APIs like BpmnElementsRegistry.getElementsByIds.

Initial proposal

export interface BpmnElement {
  bpmnSemantic: BpmnSemantic;
  htmlElement: HTMLElement;
  graphics: Graphics;
}
// Usage
graphics.updateStyle(style: StyleUpdate)
// Alternative (see below)
graphics.style.update(style: StyleUpdate)

TO BE DISCUSSED:

• review the name of the property currently named graphics and its related type
• we may also introduce a property "style" in the graphics object to later allow to get or reset the style, and provide management for CSS classes and overlays