Node FS: tracking random issues

I'm playing with the experimental Node FS API and going to use this issue for tracking random issues I'm encountering, as requested by @jasnell.

- [x] Paths with double slashes like `//bundle` resolve to just `/bundle` in Node.js on both POSIX and Windows, but result in "Error: File path must not have a host" in Workers.
- [x] The above stems from treating filenames as URLs, we should look into how other URL characters might break filesystem assumptions too (`?`, `#`, percent-encoding).
- [x] All items show up as writable, but bundled items (script itself, Wasm files etc) should probably be readonly.
- [x] All file times are reported as zeros, maybe better to either report current time or "no access" error? (captured [here](https://github.com/orgs/cloudflare/projects/52/views/1))
- [x] Lower-priority, but now that we have `process.std{in, out, err}`, we probably could expose `/dev/std{in, out, err}` file bindings too. (captured [here](https://github.com/orgs/cloudflare/projects/52/views/1))