fix: update nextjs example

[SgtPooki]

A few fixes here:

• updates all deps to latest
• updates the README.md to recommend npm run dev instead of npm start after npm install
• updates next.config.js to use polyfill recommendation based on Attempted import error: 'RTCSessionDescription' is not exported ipfs/helia#553 (comment)
  • This error cannot be reproduced with latest Helia
• removes output: 'export' from being used by default because npm run build doesn't build a proper dist folder for use by next start unless output is set to 'standalone' or undefined
• updates the test script to use next config output of 'export' so tests work properly