[examples] Switch Next.js examples to ESM, remove unnecessary type

[karlhorky]

Hi @oliviertassinari and others, first of all, thanks for Pigment CSS! I will recommend this to our students at @upleveled :)

Quick PR to fix the Next.js "manual installation" example:

1. next.config.mjs is now ESM by default since a while
2. Use common settings from create-next-app

• [x] I have followed (at least) the PR section of the contributing guide.

[oliviertassinari]

Cool. I guess we should update https://github.com/mui/pigment-css/blob/master/examples/pigment-css-nextjs-ts/ too?

[karlhorky]

• I was wrong about the Metadata import, I added it back now
• updated the example