feat: asChatEngine function for index (#1640)

.changeset/tall-kids-prove.md

examples/chat-engine/keyword-index.ts

examples/chat-engine/summary-index.ts

examples/chat-engine/vector-store-index.ts

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

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

packages/llamaindex/src/indices/BaseIndex.ts

packages/llamaindex/src/indices/keyword/index.ts

packages/llamaindex/src/indices/summary/index.ts

packages/llamaindex/src/indices/vectorStore/index.ts