present the game over scene

GameScene.swift

// omitted
class GameScene: SKScene, SKPhysicsContactDelegate {
  // omitted
  func gameOver() {
      player.removeFromParent()
      for enemy in enemies {
        enemy.removeFromParent()
      }
      // transition effect that seems like doors opening
      let reveal = SKTransition.doorsOpenHorizontalWithDuration(0.5)
      
      // create the scene
      let gameOverScene = GameOverScene(size: self.size)
      
      // present the game over scene with the transition
      self.view?.presentScene(gameOverScene, transition: reveal)
    }
  // omitted
}