WasmFS: Stable C++ and JS APIs for creating custom backends

Once we have higher confidence in the generality and robustness of our backend API, we should move the file.h header to a public location in Emscripten and document it.

A list of changes I expect we will want to make before then:

- Have an async backend API.
- Allow more backend methods to return error codes.

It would be good to complete an IndexedDB backend (#15942) and a caching virtual backend (#15940) to build confidence in the API before stabilizing it.