refactor: clean nextjs config generation (use JSON) (#631)

packages/create-llama/create-app.ts

packages/create-llama/helpers/types.ts

packages/create-llama/helpers/typescript.ts

...llama/templates/types/streaming/nextjs/next.config.app.js

...e-llama/templates/types/streaming/nextjs/next.config.json

...te-llama/templates/types/streaming/nextjs/next.config.mjs

...ma/templates/types/streaming/nextjs/next.config.static.js