fix: update formatting and DOM manipulation in SimpleCheckout.js

This commit implements minor refactoring changes in the SimpleCheckout.js file.

• Updated formatting to include space before function parameters across multiple method declarations for improved readability.
• Modified DOM manipulation logic to use data-name attribute instead of class name for element selection, enhancing selection accuracy.
• Added data-name attribute set command to retain DOM element specific data when replacing HTML.
• Improvements in 'require' callbacks to use fat arrow function notation.
• Minor adjustments in formatting in methods dealing with Events and DOM manipulations.

Related: coupons#19 (closed)