add ContextChatEngine and generator for different chat engines

create-app.ts

index.ts

templates/engines/context/constants.mjs

templates/engines/context/generate.mjs

templates/engines/context/index.ts

templates/engines/simple/index.ts

templates/index.ts

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

templates/streaming/nextjs/app/api/chat/route.ts

templates/types.ts