Refactor functions in cart.js

[rdpfeifle]

Summary

Refactor functions in cart.js to achieve better organization and a clearer separation between functionality and UI. This will enhance code readability, and maintainability, and potentially uncover opportunities to optimize or simplify the logic.

Acceptance criteria

• Thoroughly review three key functions: addToCart, updateCartQuantity, and updateProductQty.
• Refactor, delete, or separate the functionality/UI of these functions as necessary to improve clarity and efficiency.
• Ensure all console.log() statements used for debugging are removed across the application before merging the pull request.
• Test the refactored code thoroughly to confirm that the cart functionality remains intact and efficient.
• Document any significant changes or decisions made during the refactoring process.

Related issue

closes #28

Additions

• Created a function that formats the numeric price as USD currency.

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