code: function findHeader(aoa) {
    const {i} = aoa.reduce(
      (prev, row, i) => {
        const len = rowLength(row)
        if (prev.len < len) {
          return {i, len}
        }
        return prev
      },
      {i: -1, len: 0}
    )
    return i
  }
docs: finds the first row with the most columns