feat: Implement save game functionality with encoding/decoding

- Added SaveGameCodec for encoding and decoding save game files.
- Introduced SaveGamePersistence interface for slot-based save game persistence.
- Implemented FlutterSaveGamePersistence for file-based save management on Flutter.
- Enhanced WolfEngine to support saving and loading game states.
- Updated menu manager to include save/load game options.
- Created tests for SaveGameCodec to ensure proper functionality.

Signed-off-by: Hans Kokx <hans.d.kokx@gmail.com>

packages/wolf_3d_flutter/lib/wolf_3d_flutter.dart

@@ -85,6 +85,7 @@ class Wolf3d {
   WolfEngine launchEngine({
     required void Function() onGameWon,
     void Function()? onQuit,
+    SaveGamePersistence? saveGamePersistence,
     WolfRendererCapabilities? rendererCapabilities,
     WolfRendererSettings? rendererSettings,
     void Function(WolfRendererSettings settings)? onRendererSettingsChanged,
@@ -109,6 +110,7 @@ class Wolf3d {
       // so backing out of the top-level menu should not pop the route.
       onMenuExit: () {},
       onQuit: onQuit,
+      saveGamePersistence: saveGamePersistence,
       rendererCapabilities: rendererCapabilities,
       rendererSettings: rendererSettings,
       onRendererSettingsChanged: onRendererSettingsChanged,