Fix entrypoints for Next.js plugin when using Next.js app router

[jonahsnider]

Closes #235

[webpro]

Thanks @jonahsnider! Any chance you could maybe add some test coverage as well?

[jonahsnider]

Yes, I can try adding some today/tomorrow most likely.

[jonahsnider]

@webpro Added some tests, please let me know if there's anything else I should change

[webpro]

Thanks! This is great, will release shortly

[webpro]

🚀 This pull request is included in v2.24.1. See Release 2.24.1 for release notes.

[webpro]

• For next time, there's a npm run docs to regenerate docs
• I removed the next-env.d.ts as that would hide a deeper issue (Include declaration files when resolving import specifiers #206)