Add "exports" field to package.json for modern JS

[danoc]

https://web.dev/publish-modern-javascript/

I tried using exports but had to remove it in #33 to fix #31. It'd be great to add back since it's smaller than the other bundles. I'm not sure if the issue was in Next.js or microbundle, but it should be easy to find out by testing Next.js with any other package that uses exports in package.json.