Add download progress callback

[ngxson]

Supersed #8

Usage:

await wllama.loadModelFromUrl(MODEL_SPLITS, {
  embeddings: true,
  n_ctx: 1024,
  parallelDownloads: 5,
  progressCallback: ({ loaded, total }) => console.log(`Downloading... ${Math.round(loaded/total*100)}%`),
});

Please note that this also works with multiple splits (the reported progress will be aggregated, that means it reports the global progress across all files)