refactor: move chat engine & retriever into core (#1242)

.changeset/four-beers-kick.md

README.md

examples/agent/azure_dynamic_session.ts

examples/agent/stream_openai_agent.ts

examples/agent/wiki.ts

examples/anthropic/chat_interactive.ts

examples/chatEngine.ts

examples/chatHistory.ts

examples/cloud/chat.ts

examples/multimodal/context.ts

examples/multimodal/rag.ts

packages/autotool/examples/02_nextjs/actions.ts

packages/core/package.json

packages/core/src/chat-engine/index.ts

packages/core/src/global/settings/callback-manager.ts

packages/core/src/query-engine/base.ts

packages/core/src/retriever/index.ts

...maindex/e2e/examples/cloudflare-worker-agent/src/index.ts

...lamaindex/e2e/examples/nextjs-agent/src/actions/index.tsx

packages/llamaindex/e2e/node/openai.e2e.ts