add conditional exports

[cometkim]

No description provided.

[remcohaszing]

This PR breaks native ESM (.mjs extension is necessary) and TypeScript (CJS interface does not match ESM interface, although this was already broken) support. Also there there should be a "default" field, which should be the last key in the export map. Since this package doesn’t specify "type": "module", the file to resolve for require should he marked as "default".

I proposed a proper fix in #57, but the author has shown no interest to resolve this issue. This does not include exports for package.json as you added here.