WasmFS: Asyncify support for async backends

As mentioned in https://github.com/emscripten-core/emscripten/issues/15949#issuecomment-1015089616. I'm not sure what we'd have to do to make Asyncify work correctly to provide a sync interface to async backends, if anything. I'm also not sure whether this would be a new feature or if the current file system already supports this.