chore(docs): Add jsdoc/tsdoc to methods

[bdellis]

Add jsdocs to methods in the repo - this is a first stab to get feedback from the team.

[gregjopa]

I think it would be great to explain how this function gets invoked. What do you think of this?

/**
 * Initializes the PayPal Web SDK, determines eligible payment methods,
 * and sets up the appropriate payment buttons. This function is invoked
 * by the SDK script tag's "onload" event.
 *
 * ```html
 * <script
 *   async
 *   src="https://www.sandbox.paypal.com/web-sdk/v6/core"
 *   onload="onPayPalWebSdkLoaded()">
 * </script>
 * ```
 *
 * @async
 * @function
 * @returns {Promise<void>}
 */

[gregjopa]

I was reading up on jsdoc and it looks like we can use typedef to type out and reuse this objects. Would be great to leverage to explain what's available in each object:

/**
 * @typedef {Object} OnApproveData
 * @property {string} orderId
 * @property {string} payerId
 */

/**
 * @typedef {Object} OnCancelData
 * @property {string|undefined} orderId
 */

/**
 * Options object for payment session event handlers.
 * @typedef {Object} PaymentSessionOptions
 * @property {function(OnApproveData): Promise<void>} onApprove - Called when the buyer approves the payment.
 * @property {function(OnCancelData): void} onCancel - Called when the buyer abandons the payment.
 * @property {function(Error): void} onError - Called when an unexpected error occurs.
 */

[gregjopa]

I'm learning that we can type out object properties like this too:

Suggested change

	* @param {Object} paylaterPaymentMethodDetails - The Pay Later payment method details.
	* @param {{ productCode: string, countryCode: string }} paylaterPaymentMethodDetails - The Pay Later payment method details.

[gregjopa]

@bdellis this is an incredible start with leveraging code comments to better explain our examples. I left some comments with some ideas.