[Experimental Feature] Add support for cross-origin storage

[akaashrp]

1. Add support for cross-origin storage caching
2. Deprecate useIndexedDBCache

See apache/tvm#18893

[tomayac]

This looks great. I just left a couple of suggestions, mostly minor cosmetic things like updating links and such.

Here's an additional suggestion for what you could add to the docs at docs/user/advanced_usage.rst (line 119):

Using Cross-Origin Storage Cache
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

WebLLM also supports caching model artifacts across different origins using the experimental Cross-Origin Storage API. You can enable this cache backend by setting ``appConfig.cacheBackend = "cross-origin"``. For users with the `Cross-Origin Storage browser extension <https://chromewebstore.google.com/detail/cross-origin-storage/denpnpcgjgikjpoglpjefakmdcbmlgih>`_ installed, resources will then be cached and shared across origins. This means two independent apps opted into this cache backend using the same AI model will download and cache the required resources only once. See the `cache usage example <https://github.com/mlc-ai/web-llm/tree/main/examples/cache-usage>`_ for more details. If Cross-Origin Storage isn't available, WebLLM will automatically fall back to using the default cache.

.. code-block:: typescript

   const engine = await CreateMLCEngine("Llama-3.1-8B-Instruct", {
       appConfig: {
           cacheBackend: "cross-origin",
       },
   });

[tomayac]

Amazing, thank you very much for landing this! ♥️