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

packages/create-llama/index.ts

packages/create-llama/templates/index.ts

...create-llama/templates/simple/nextjs/app/api/llm/route.ts

...create-llama/templates/simple/nextjs/app/api/route/llm.ts

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

packages/create-llama/templates/simple/nextjs/app/layout.tsx

packages/create-llama/templates/simple/nextjs/app/page.tsx

packages/create-llama/templates/simple/nextjs/next.config.js