refactor(core): move `ContextChatEngine` and `SimpleChatEngine` (#1401)

.changeset/shaggy-houses-breathe.md

apps/next/src/actions/index.tsx

packages/core/src/agent/base.ts

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

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

packages/core/src/chat-engine/default-context-generator.ts

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

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

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

packages/llamaindex/src/engines/chat/index.ts