Allow level props to have a flipX field in JSON

source/funkin/data/story/level/LevelData.hx

@@ -114,4 +114,11 @@ typedef LevelPropData =
   @:default([])
   @:optional
   var animations:Array<AnimationData>;
+
+  /**
+   * Flips the sprite on X axis.
+   */
+  @:default(false)
+  @:optional
+  var flipX:Null<Bool>;
 }

source/funkin/ui/story/LevelProp.hx

@@ -78,6 +78,7 @@ class LevelProp extends Bopper
     this.alpha = propData.alpha;
     this.x = propData.offsets[0];
     this.y = propData.offsets[1];
+    this.flipX = propData.flipX;
 
     FlxAnimationUtil.addAtlasAnimations(this, propData.animations);
     for (propAnim in propData.animations)