save ETag metadata, add allowOffline

[ngxson]

Add allowOffline as an option for loadModelFromUrl. If model is in cache and fetch request fails (for any reasons), then we fallback to cached model file:

await wllama.loadModelFromUrl(MODEL, {
  allowOffline: true,
}

This should work without relying on Navigator.onLine

---

A method is also added to retrieve data for cached file:

await wllama.cacheManager.getMetadata(url); // null if file not in cache
// or, list all files with metadata
await wllama.cacheManager.list();

[nfplay]

Hi, this works on desktop if network is not present, but i'm not getting the same experience on mobile in airplane mode for instance. Are you able to reproduce the same behaviour on mobile?

Thanks,
Nuno