Naming convention updates

[wozozo]

This pull request primarily focuses on standardizing naming conventions across the codebase by updating file and import names from singular to plural forms. Additionally, it removes an outdated dependency reference from the documentation. These changes improve consistency and maintainability.

Naming convention updates:

• Renamed import statements in src/index.ts to use pluralized file names, such as accounts, balances, charges, customers, etc., instead of their singular forms.
• Renamed the file src/customer.ts to src/customers.ts and updated the import from ./card to ./cards to match the plural naming convention.

Documentation update:

• Removed the reference to the superagent dependency from the README.md file, as it is no longer relevant.

close #66