EXTRA: Currency System

**ONLY ATTEMPT ONCE ALL OTHER MECHANICS ARE COMPLETE!**

- A player should receive some form of currency upon defeating an enemy.
- Currency can be spent in the shop. (See the issue "EXTRA: Shop System" for more information on the shop)
- Currency should be displayed as a GUI element in addition to the ones described in the issue "UI Elements and HUD".
- Currency should be reset upon restarting, player death, or successful completion of the game.
- Currency should be decreased upon buying an item.

此 issue 未指定任何檔案或測試，並且依賴其他機制，尤其是「EXTRA: Shop System」和「UI Elements and HUD。」這些系統完成後，在 Unity 專案中追蹤擊敗敵人、商店購買、HUD、重新開始、死亡和遊戲完成流程。完成條件是：在列出的每種情況下，貨幣都會被發放、顯示、花費並重設。

由索引模型根據 Issue 內容生成。