Fix exports for some bundlers

[proudparrot2]

I've been using this package in a javascript library with esbuild and it's worked fine, but for some reason, rspack can't resolve it. Adding "main": "index.js" to package.json fixed this.

[wesleytodd]

While this is a good change and likely to land here, you might want to report that as a bug to them. The default behavior when main is missing is to use index.js: https://nodejs.org/docs/latest/api/modules.html#folders-as-modules