[Cache] Add cachedPrefixes for caching repeated system prompts

[YiyanZhai]

This PR adds the cachedPrefixes field in MLCEngineConfig, allowing users to cache system prompts when creating MLCEngine. It reduces redundant processing of repeated instructions.

Example usage in CreateMLCEngine:

await webllm.CreateMLCEngine(
  selectedModel,
  {
    initProgressCallback: initProgressCallback,
    logLevel: "INFO",
    cachedPrefixes: [
      [ { role: "system", content: "You are a helpful assistant running in the user's browser. You need to answer questions ... " }, ]
    ],
  },
  {
    context_window_size: 2048,
  }
);