Support web platform

[hcanyz]

fix #3.

currently only available, performance is of magnitude worse than native.

I ran a rough performance test, use the following test method:

void main() {
  _benchmark();
}

void _benchmark() {
  final startTime = DateTime.now().millisecondsSinceEpoch;

  final Set<int> history = {};

  int conflictedCount = 0;
  for (int index = 0; index < 3000000; index++) {
    final id = createUniqueId();
    final exist = history.contains(id);
    if (exist) {
      debugPrint("$id $index conflicted.");
      conflictedCount++;
    }
    history.add(id);
  }

  debugPrint(
      "$conflictedCount ${history.length} ${DateTime.now().millisecondsSinceEpoch - startTime}");
}

int createUniqueId() {
  final fnv = fnv1a_64_s_bigint(const Uuid().v4());
  return (fnv & BigInt.from(0x1FFFFFFFFFFFFF)).toInt();
}

For the same device, it takes about 3 seconds for 3 million pieces of data on native, but it takes about 3 seconds for 30,000 pieces of data on the web platform.

Of course, there is a lot of time here to do other events, such as generating uuid, this is just a rough test.

[hcanyz]

@Midi12 Can it be merged? : )

[Midi12]

@hcanyz Thank you for your contribution, I will be able to review it soon, I set up a reminder. I will notify you, thank you for your patience.