add test form for nextjs simple (and make generation work)

index.ts

templates/index.ts

templates/simple/nextjs/app/api/llm/route.ts

templates/simple/nextjs/app/api/route/llm.ts

templates/simple/nextjs/app/components/message-form.tsx

templates/simple/nextjs/app/layout.tsx

templates/simple/nextjs/app/page.tsx

templates/simple/nextjs/next.config.js