Support for m4a files with mediaelement

Problem/Motivation

I would like for this module to support m4a audio files with mediaelement library. And I could achive this by adding another fileType in audiofield/src/Plugin/AudioPlayer/MediaElementAudioPlayer.php. It looks like this:

*   fileTypes = {
 *     "mp3", "m4a", "oga", "ogg", "wav",
 *   },