feat: Add showSubtitlesPerDefault flag to control subtitles (fluttercommunity#648)

Introduced `showSubtitlesPerDefault` in `ChewieController` to allow disabling subtitles by default. Updated controls to respect the flag and aligned behavior across platforms. Added documentation.

Closes fluttercommunity#648

Author: floodoo

example/lib/app/app.dart

@@ -42,8 +42,8 @@ class _ChewieDemoState extends State<ChewieDemo> {
 
   List<String> srcs = [
     "https://commondatastorage.googleapis.com/gtv-videos-bucket/sample/BigBuckBunny.mp4",
-    'https://commondatastorage.googleapis.com/gtv-videos-bucket/sample/ElephantsDream.mp4',
-    'https://commondatastorage.googleapis.com/gtv-videos-bucket/sample/ForBiggerBlazes.mp4',
+    "https://commondatastorage.googleapis.com/gtv-videos-bucket/sample/ElephantsDream.mp4",
+    "https://commondatastorage.googleapis.com/gtv-videos-bucket/sample/ForBiggerBlazes.mp4",
   ];
 
   Future<void> initializePlayer() async {