Manually provide `814.ffmpeg.js`'s path

**Is your feature request related to a problem? Please describe.**
I cannot load the module in a Chrome extension scaffolded by [Plasmo](/plasmohq/plasmo)
The reason is that when FFmpeg loads up, it looks for `814.ffmpeg.js` in the same directory as `ffmpeg.js`, whereas Plasmo, which uses [Parcel](/parcel-bundler/parcel) under the hood, renames the file to include a hash, e.g. `814.ffmpeg.d7h8ng.js`

**Describe the solution you'd like**
Allow manually providing a path to `814.ffmpeg.js`

**Describe alternatives you've considered**
The only alternative is to create an extension without using Plasmo