fix: modal exports for node16

[connorads]

I have a type checking issue in my repository, similar to the one described in #4991 (comment) with @mantine/modals/lib/index.d.mts
Which was then fixed in #5697

I couldn't get the paths workaround working, so to solve this locally I can

1. Delete@mantine/modals/lib/index.d.mts in postinstall script and it will rely on the non-module file @mantine/modals/lib/index.d.ts
2. Or, patch the file to add .js to exports
   

This PR makes the change to do 2.
Tbh I don't really know what I'm doing, I suspect we don't want to do this because I believe it will also add .js to the non-module @mantine/modals/lib/index.d.ts file which we probs don't want.

Was hoping that @waweber might be able to help 🙏

[rtivital]

Thanks!

[connorads]

No problem!

I was hoping that @waweber might validate the solution 😅 But if you're happy with it then great 👍