fix: support positional callbacks in initializePayment

[DevMubarak1]

Resolves #101.

Context

In use-paystack.ts, initializePayment currently destructures { config, onSuccess, onClose } directly from the first argument.

The README and earlier versions of this library show callers passing positional arguments:

initializePayment(onSuccess, onClose);

When called this way, onSuccess is passed as the first parameter (the options object), and destructuring { config, onSuccess, onClose } yields undefined for both callbacks. The payment modal opens, but neither callback ever fires. Calling initializePayment() without arguments also throws a TypeError when destructuring undefined.

What this PR does

• Checks whether the first parameter is a callback function or an options object.
• If a function is passed, it treats arguments positionally: (onSuccess, onClose, config).
• If an object is passed, it continues reading { config, onSuccess, onClose }.
• If called with no arguments (initializePayment()), it handles it cleanly without throwing.
• Overloads the InitializePayment type in libs/types.ts so TypeScript users get accurate typing and autocomplete for both patterns.
• Adds test cases in libs/test/use-paystack.test.tsx covering positional callbacks and zero-argument calls.

Both styles work as expected now and existing code using the object syntax won't break.