[REQUEST] ESM version of Dataloader

## What problem are you trying to solve?
The library is 6 years old and could benefit from using modern ES features, plus ESM is easier to tree-shake. I believe I had some TypeScript issues as well -- dataloader is built using Flow so it requires more maintenance.

## Describe the solution you'd like

Implementation: https://github.com/macalinao/grill/tree/master/packages/dataloader-es

I've also ported over all tests. I also exclusively use `setTimeout` instead of checking for environment-- not sure if this will cause issues.

## Describe alternatives you've considered

N/A

## Additional context

https://github.com/macalinao/grill/tree/master/packages/dataloader-es