Cannot execute ffmpeg.FS('init', {...}) successfully

**Describe the bug**
If I try to execute `ffmpeg.FS('init', {...})` so I can use stdin and stdout with ffmpeg, the code doesn't work. I get
> Oops, something went wrong in FS operation.

And no more information of what went wrong.

[FS.init()](https://emscripten.org/docs/api_reference/Filesystem-API.html#setting-up-standard-i-o-devices) is documented in emscripten and the [ffmpeg.wasm API documentation ](https://github.com/ffmpegwasm/ffmpeg.wasm/blob/master/docs/api.md#ffmpeg-fs) tells both:

> Run FS operations.

> For more info, check https://emscripten.org/docs/api_reference/Filesystem-API.html

**To Reproduce**
Place all ffmpeg files (including `ffmpeg.min.js`) in the current directory
Use this HTML:
```

Document

(async () => {
const { createFFmpeg, fetchFile } = FFmpeg;
const ffmpeg = createFFmpeg({ log: true });
await ffmpeg.load();
ffmpeg.FS('init', {
stdin: () => {
return null
}
});
})()

```
When it's done loading, it will fail with the error.

**Expected behavior**
No exception happens and emscripten's `FS.init` is executed with the provided object (which would allow streaming video content)

**Screenshots**
If applicable, add screenshots to help explain your problem.

**Desktop (please complete the following information):**
- OS: N/A
- Browser firefox
- Version 107

**Additional context**

I'm trying to stream data from the website to ffmpeg.
If I use an url for input, a websocket is opened at the root of the URL and raw HTTP traffic is sent through it. That would require custom code to handle the traffic (not useful! I need the root path for regular HTTP traffic; not websockets! If I can control the path where the websocket is requested, then I could do something about it!).
If I try to stream the input into stdin, I only get complications with it because I need to provide data byte by byte. Even then I tried continuing. I get this bug I'm reporting.

I try to figure out how to have an uncompressed version because variable names don't match but I can't find instructions on how to generate `ffmpeg.js` without being minimized. Given so, I can't help you beyond what I have here.