add ContextChatEngine and generator for different chat engines

packages/create-llama/create-app.ts

packages/create-llama/index.ts

...ages/create-llama/templates/engines/context/constants.mjs

packages/create-llama/templates/engines/context/generate.mjs

packages/create-llama/templates/engines/context/index.ts

packages/create-llama/templates/engines/simple/index.ts

packages/create-llama/templates/index.ts

...reate-llama/templates/simple/nextjs/app/api/chat/route.ts

...te-llama/templates/streaming/nextjs/app/api/chat/route.ts

packages/create-llama/templates/types.ts