[Request] Add support for Bauer stereophonic-to-binaural (bs2b)

[Farnjang]

What do you want to be added?

Support for Bauer stereophonic-to-binaural DSP.

Not sure how to add it. I think ffmpeg (where is it?) needs --enable-libbs2b with https://sourceforge.net/projects/bs2b/files/libbs2b/3.1.0/.

Then on Feishin options it could be enabled by using --af-add=lavfi=[bs2b=profile=default].

Additional context

BS2B reduces the 'stereo fatigue' one can experience when listening to music with headphones for extended periods of time