[REFACTOR] Use the new `bv-experimental-add-ons` API using "name" parameters

[assynour]

Currently (b1d8e33), the demo uses custom code to get elements by name, then the retrieved ids are passed to the API of bpmn-visualization.

We could use the API provided by bv-experimental-add-ons that directly use names as parameters:

• [FEAT] Introduce a plugin to expose the bpmn-visualization "CSS classes API" with names as parameters bpmn-visualization-addons#73
• [FEAT] Introduce a plugin to expose the bpmn-visualization "overlays API" with names as parameters bpmn-visualization-addons#74
• [FEAT] Introduce a plugin to expose the bpmn-visualization "style API" with names as parameters bpmn-visualization-addons#75
• [FEAT] Introduce a plugin to expose the bpmn-visualization "elements API" with names as parameters bpmn-visualization-addons#76