Add support for audio muxing

[bil-ash]

Thanks for this awesome library.

Now, since webcodecs also supports aac encoding (at least on desktop), please add support for audio muxing so that we can generate proper mp4 video files (i.e., with both audio and video) using this library which can be played on any media player

[Vanilagy]

@bil-ash Hey, I just released my own library to mux MP4 files, which has audio support and doesn't use WASM at all: https://github.com/Vanilagy/mp4-muxer