Possible issue with missing type declaration files for ESM exports

Hi, this is possibly user error, but a recent routine release of my lib [react-flip-toolkit](https://github.com/aholachek/react-flip-toolkit) broke type declarations-- a user filed an [issue here](https://github.com/aholachek/react-flip-toolkit/issues/224) and when I view the analysis [on this page](https://arethetypeswrong.github.io/?p=react-flip-toolkit%407.2.0) I see that types are not found.

After digging around for a bit, it seems possible based on my reading of [this explanation](https://github.com/arethetypeswrong/arethetypeswrong.github.io/blob/main/docs/problems/UntypedResolution.md) that we need to package an `index.d.mts` alongside the `index.d.ts` automatically generated by microbundle.