serialize(isForCopying = false, context = null) {
    unreachable("An editor must be serializable");
  }
Serialize the editor.
The result of the serialization will be used to construct a
new annotation to add to the pdf document.

To implement in subclasses.
@param {boolean} [isForCopying]
@param {Object | null} [context]
@returns {Object | null}