insertTab(rng, tabsize) {
    const tab = dom.createText(new Array(tabsize + 1).join(dom.NBSP_CHAR));
    rng = rng.deleteContents();
    rng.insertNode(tab, true);

    rng = range.create(tab, tabsize);
    rng.select();
  }
insert tab

@param {WrappedRange} rng
@param {Number} tabsize