[Web][PROPOSAL]: implement ArtifactCache using OPFS

[KarolNov]

Motivation
OPFS has better persistence, performance, and consistent support for large files across browsers.
Personally I would like to see this caching strategy for https://github.com/mlc-ai/web-llm.

This PR doesn't implement all required changes and hasn't been proved to work yet - I just created a PoC class implementing the ArtifactCacheTemplate.
Potentially a problem with this approach would be that this cache uses OPFS on main thread with async APIs. A better solution could be to use Web Worker and sync file handlers - https://developer.mozilla.org/en-US/docs/Web/API/File_System_API/Origin_private_file_system#manipulating_the_opfs_from_a_web_worker. Both approaches could be valid for different use cases, so I propose creating 2 new caching strategies - opfs and opfs-worker.
I could finish the required work if we find it useful. Otherwise I would just move caching higher up in my stack and just do it for my specific use case.

[KarolNov]

I'm new to this repo, but it seems versioning is not really needed here and it's up for user to version the urls instead. However storing the -vX suffix would allow to do internal breaking changes in the file structure by just creating a new OPFS cache handler that uses different dir name -v2.