The spec for this action:
https://www.adobe.com/content/dam/acom/en/devnet/pdf/pdfs/PDF32000_2008.pdf#G11.2111850
An example of a form using this action:
https://www2.illinois.gov/rev/forms/reg/Documents/reg-1-mr.pdf
selectElement.multiple = true;
          },
          remove(event) {
            const options = selectElement.options;
            const index = event.detail.remove;
            options[index].selected = false;
            selectElement.remove(index);
            if (options.length > 0) {
              const i = Array.prototype.findIndex.call(
                options,
                option => option.selected
              );
              if (i === -1) {
                options[0].selected = true;
              }
            }
            storage.setValue(id, {
              value: getValue(event,  true),
              items: getItems(event),
            });
          },
          clear(event) {