Feature/add stars renderer (#70)

Co-authored-by: Djelal BOUDJI <djelal@gmail.com>

Assets/Resources/Levels/BackOnTrack.json

@@ -2,6 +2,7 @@
     "name": "Back on Track",
     "musicName": "BackOnTrack",
     "order": 2,
+    "difficulty": 1,
     "elements": [
         {
             "type": "Spike",

Assets/Resources/Levels/StereoMadness.json

@@ -2,6 +2,7 @@
     "name": "Stereo Madness",
     "musicName": "StereoMadness",
     "order": 1,
+    "difficulty": 2,
     "elements": [
         {
             "type": "Spike",