WasmFS: Pre-open OPFS files

For the OPFS backend in particular, reading and writing (and potentially other operations, depending on what happens with the APIs) are synchronous, but it takes some asynchronous work to acquire the necessary handles first. If we could "pre-open" files on a main application thread by doing that asynchronous work during startup, then we would be able to synchronously read and write the files from that thread without any further proxying.