Feature Request: Mount OPFS subdirectories

I am building a multi-module website that would benefit from separate mount points for each module.

I propose the ability to mount OPFS subdirectories in wasmfs. Currently, only the OPFS root can be mounted. The backend returned by wasmfs_create_opfs_backend() always binds to the root.

I suggest adding something like `wasmfs_create_opfs_backend_at(path)` which takes a path and returns a backend representing a subdirectory of the OPFS root.

If requested, I can provide an initial implementation of this feature.