Checkpoint 5 _ Polish and UI

In my work this week, I made the following changes to the game: added the opening music and the sound effects produced by bullets hitting the enemy during the game, and adopted the suggestions of my classmates, and added the background of the game story before the start page The introduction and the operation method of the game, they can all enter the next link by pressing the space bar. At the same time, I also added a restart button during the game, so that the player can choose to restart the game if they feel unble to pass the level.