function readZigZagBigInt(reader) {
  const zigzag = readVarBigInt(reader)
  // convert zigzag to int
  return zigzag >> 1n ^ -(zigzag & 1n)
}
A zigzag int folds positive and negative numbers into the positive number space.
This version returns a BigInt.

@param {DataReader} reader
@returns {bigint}