function readVarBigInt(reader) {
  let result = 0n
  let shift = 0n
  while (true) {
    const byte = reader.view.getUint8(reader.offset++)
    result |= BigInt(byte & 0x7f) << shift
    if (!(byte & 0x80)) {
      return result
    }
    shift += 7n
  }
}
Read a varint as a bigint.

@param {DataReader} reader
@returns {bigint}