Add game props: Shrink Potions

[2021110706]

Issue Description:
In the current game logic, we believe that adding game props can make the game more interesting. Props can be randomly spawned during the game, providing players with additional challenges and rewards.

Proposed Solution:
We propose adding a new folder to the project dedicated to storing various props' code and resources. By implementing corresponding logic in the game element layer, we can randomly spawn these props during the game.

Related Discussion:
Our previous conversation has covered the logic of adding a shrink potion. Now, we aim to expand this logic to include more types of game props.

Steps or Plan:

1. Create a new folder to store the code and resources for game props.
2. Add logic in the game element layer to randomly spawn props during the game.
3. Define the effects of different props, such as the shrink potion reducing the bird's size.

Future Additions:
Subsequently, more props can be added, such as invisibility potions (making the bird invincible during invisibility), one-time-use defensive shields, etc.

Discussion and Feedback:
We look forward to your feedback and suggestions to better integrate this feature into the project.