fix js distributable

[bakkot]

#6048 reorganized the files in the JS api which made the published artifact not work (so the just-published 4.9.0 doesn't work at all; attempting to import it throws). This fixes that. It also marks the package as usable on node 18, due to the inclusion of #6136.

[NikolajBjorner]

thanks for looking into this.
Let me know if I need to crank up 4.9.1 today or we can revert the npmjs dist.

[bakkot]

It's possible to npm unpublish the new version, so that only the old version exists, but unfortunately you can't just then re-publish the fixed thing - npm won't let you reuse a version number even after unpublishing. It will require a new version to get something which works and also is current.

[NikolajBjorner]

so I will then roll a 4.9.1 now

[bakkot]

4.9.1 looks like it works! 🎉