function readCollectionBegin(reader) {
  const sizeType = reader.view.getUint8(reader.offset++)
  const size = sizeType >> 4
  const type = getCompactType(sizeType)
  if (size === 15) {
    const newSize = readVarInt(reader)
    return [type, newSize]
  }
  return [type, size]
}
Read collection type and size

@param {DataReader} reader
@returns {[number, number]} [type, size]