refactor(core): move `ContextChatEngine` and `SimpleChatEngine` #5887

Job	Run time
E2E on Node.js 18.x	3m 0s
E2E on Node.js 20.x	2m 55s
Build LlamaIndex Example (cloudflare-worker-agent)	2m 27s
Test on Node.js 18.x	2m 39s
E2E on Node.js 22.x	2m 53s
Build LlamaIndex Example (nextjs-agent)	3m 22s
Test on Node.js 20.x	2m 31s
Build LlamaIndex Example (nextjs-edge-runtime)	2m 53s
Test on Node.js 22.x	2m 27s
typecheck	2m 47s
typecheck-examples	3m 12s
Build LlamaIndex Example (nextjs-node-runtime)	3m 5s
Build LlamaIndex Example (waku-query-engine)	2m 45s
Build LlamaIndex Example (llama-parse-browser)	2m 26s
	39m 22s

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