These changes make it easier to integrate vorbisfile and opusfile from
source.

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.

Public issue https://github.com/mackron/miniaudio/issues/942

This is in preparation for splitting miniaudio into a split .c/h pair,
away from a single header.

`MINIAUDIO_IMPLEMENTATION` is still supported, but will be removed in
version 0.12 and should be considered deprecated. It's recommended to
start the transition to the new .c file.

Revert "Fix for NDK issue 360. dlclose() skipped pre-API 28. This fixes weird crashes during uninit."

This reverts commit 14f698fcf289261946aefd0000dc5989062cf423.