Provide multiple wasms with different encoder sets

**Is your feature request related to a problem? Please describe.**
`ffmpeg-core.wasm` is about 25 MB, and can take a very long time to load, especially on mobile connections.

**Describe the solution you'd like**
Provide alternate wasm builds that contain only a few choice encoders and some popular decoders.
This way, if you know you're only going to produce x264-encoded files, you only have to load the encoders you need.
This saves bandwidth and time. This is already done in libraries such as [Kagami/ffmpeg.js](https://github.com/Kagami/ffmpeg.js).

**Describe alternatives you've considered**
You could also try to make the wasm build smaller without losing functionality, but I don't think there's much room to improve there.

**Additional context**
N/A