next-env.d.ts は gitignore する
公式もこう言っている。
Running next dev, next build, or [next typegen](https://nextjs.org/docs/app/api-reference/cli/next#next-typegen-options) regenerates this file.
Good to know:
We recommend adding next-env.d.ts to your .gitignore file.
The file must be in your tsconfig.json include array (create-next-app does this automatically).