Add default export to package.json

[wesbos]

Seems some bundlers (webpack, turbopack) can't import this when the rehype package is passed as a string.

Tracking the issue here, but adding the default key also fixes it.

Same issue this package, the merge fixed it. stefanprobst/rehype-extract-toc#10

tracking the issue: vercel/next.js#73757

Would appreciate if you could merge + cut a release.

[jrson83]

@wesbos awesome thank you! I will update deps and release a new version.

[wesbos]

thanks a ton!