Support ESM

[sindresorhus]

I have started to convert some of my packages to ESM and I noticed an issue:

index.test-d.ts:4:0
  ✖  2:7  Module "/Users/sindresorhus/dev/oss/array-union/index" can only be default-imported using the esModuleInterop flag

I think we should set esModuleInterop in tsd for now to work around TS's lack of ESM support.