add package.json exports

[sagargurtu]

Adding exports field in package.json to make it easier for modern bundlers to properly bundle this package.

[ljharb]

Adding exports is always a breaking change, and I'm not sure why a modern bundler would have any issue with a package that lacks exports (especially since that's most of them)

[sagargurtu]

Apologies - I should have been clearer in the initial description. I'm currently developing a Vite plugin that relies on custom resolution logic and I noticed that this line disrupts that behavior. I thought this could be a good opportunity to introduce the exports field in package.json, as it's a more modern and explicit approach for defining entry points.

I do realize this would be a breaking change, but I'm happy to ensure all entry points are properly mapped in the structure to maintain compatibility. If the necessary non-breaking changes are made, would this pull request be good to go, or would you prefer that I withdraw this change entirely? I'm happy to follow whichever direction works best.

[ljharb]

If your plugin can't handle arbitrary paths in an exports-less package, then it's broken. While I definitely would want to eventually add exports to this package, I don't think this is a sufficiently motivating rationale.

[sagargurtu]

Sounds good. Closing this PR.