These decoders have been moved into their own subfolders under the
extras/decoders folder:

  extras/decoders/libvorbis
  extras/decoders/libopus

In addition to being relocated, they have also been split into separate
.c/h pairs. They now work like a more conventional library. The
implementation of these libraries have also been decoupled from the
miniaudio implementation which means they depend only on the header
section of miniaudio.h now.

With this change the custom_decoder and custom_decoder_engine examples
have been updated. To compile these you now need to link in the
miniaudio_libvorbis.c and miniaudio_libopus.c files via your build
tool. For your own code, you can still include the .c files directly
into your code if you want to compile as a single translation unit.