[examples] Update Next.js example to use ESM

[oliviertassinari]

Summary

pigment-css/examples/pigment-css-nextjs-ts/next.config.js

Line 1 in b4e2d77

const { withPigment, extendTheme } = require('@pigment-css/nextjs-plugin');

is using CJS. But npx create-next-app@latest gives you a next.config.mjs file. So we need to update this example.

Motivation

Make it easier to copy and paste the example to an existing project.