Changes the opacity of an arbitrary cell view's SVG node.
When a cell is highlighted with the opacity highlighter, the node determined by the selector is given the
.joint-highlight-opacity class. To customize the look of this highlighted state, you can add custom CSS rules that affect this class name.
This highlighter does not currently have any options.
joint.highlighters.opacity.add(cellView, 'body', 'my-highlighter-id');