EmbedThumbnail - support m4v extention

Some platforms can serve m4v for mpeg video, this commit simply
adds support for embedding thumbnails into m4v files.

Tested with both AtomicParsley and Mutagen. It is possible that
m4v files could include DRM, but I don't have any DRM m4v files
to test on. The existing error handling should probably be fine
in those cases.

yt_dlp/postprocessor/embedthumbnail.py

@@ -114,7 +114,7 @@ def run(self, info):
             self._report_run('ffmpeg', filename)
             self.run_ffmpeg(filename, temp_filename, options)
 
-        elif info['ext'] in ['m4a', 'mp4', 'mov']:
+        elif info['ext'] in ['m4a', 'mp4', 'm4v', 'mov']:
             prefer_atomicparsley = 'embed-thumbnail-atomicparsley' in self.get_param('compat_opts', [])
             # Method 1: Use mutagen
             if not mutagen or prefer_atomicparsley:
@@ -213,7 +213,7 @@ def run(self, info):
             temp_filename = filename
 
         else:
-            raise EmbedThumbnailPPError('Supported filetypes for thumbnail embedding are: mp3, mkv/mka, ogg/opus/flac, m4a/mp4/mov')
+            raise EmbedThumbnailPPError('Supported filetypes for thumbnail embedding are: mp3, mkv/mka, ogg/opus/flac, m4a/mp4/m4v/mov')
 
         if success and temp_filename != filename:
             os.replace(temp_filename, filename)