Add the split version of miniaudio to the extras folder.

extras/miniaudio_split/README.md

+These files split the main library into separate .h and .c files. This is intended for those who prefer separate files
+or whose build environment better suits this configuration. The files here are generated from a tool based on the
+content in the main miniaudio.h file. Do not edit these files directly. If you want to contribute, please make the
+contribution in the main file.
+
+This is not always up to date with the most recent commit in the dev branch, but will usually be up to date with the
+master branch.
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