An index signature parameter type cannot be a union type

**Describe the bug**
After yarn add @ffmpeg/ffmpeg @ffmpeg/util
I receive the following error just trying to build my angular app:

```
ERROR: node_modules/@ffmpeg/ffmpeg/dist/esm/types.d.ts:121:6 - error TS1337: An index signature parameter type cannot be a union type. Consider using a mapped object type instead.

121 [id: number | string]: (data: CallbackData) => void;
```

https://github.com/ffmpegwasm/ffmpeg.wasm/blob/147596bdb08848cf53af2cd1afb9e1c0745721fa/packages/ffmpeg/src/types.ts