HLS: Hook on closing a chunk/file

Hello. I'm using ffmpeg.wasm to create a HLS playlist from a given input.
I want to upload the individual chunks/files (`stream0.ts`, `stream1.ts`, `...`) asynchronously - to be more precise - as soon as ffmpeg has finished writing them.

I'm unsure on how to achieve this. ffmpeg doesn't seem to log when it's closing a file, only when opening:

> [fferr] [hls @ 0x1a7b5c0] Opening '/output/stream1.ts' for writing

This of course isn't the information I need because I may only start uploading a file once it's done being written to. I could infer that if I encounter the log message

> [fferr] [hls @ 0x1a7b5c0] Opening '/output/stream1.ts' for writing

that `stream0.ts` (`n-1`) is ready for upload, but this is quite hacky in my opinion plus there are edge cases like the playlist files and so on.

I also thought about continiously reading the FS with

```js
ffmpeg.FS('readdir' as FSMethodNames, '/output')
```

But I have the same problem. files appear as soon as ffmpeg opens them for writing.

Looking through the Emscripten FS API [this](https://emscripten.org/docs/api_reference/Filesystem-API.html#FS.trackingDelegate[callback%20name]) seems to be exactly what I need:

```js
FS.trackingDelegate['onCloseFile'] = function(path) {
out('Closed ' + path);
};
```

However, I don't really understand an how to use that inside my React application. This is my first time working with ffmpeg and WASM in general. I'd appreciate any help.

Thanks.