The quantity in the checkout event tracking is misplaced as index value

[eason9487]

Describe the bug:

Found this issue while working on #341.

In trackCheckoutStep function, it maps the cartItems array to getProductFieldObject.

woocommerce-google-analytics-integration/assets/js/src/tracking.js

Lines 100 to 109 in 64e7dda

	export const trackCheckoutStep =
	( step ) =>
	( { storeCart } ) => {
	if ( currentStep === step ) {
	return;
	}
	trackEvent( step === 0 ? 'begin_checkout' : 'checkout_progress', {
	items: storeCart.cartItems.map( getProductFieldObject ),
	coupon: storeCart.cartCoupons[ 0 ]?.code || '',

However, the second parameter of getProductFieldObject is used as the quantity tracking property rather than the array index.

woocommerce-google-analytics-integration/assets/js/src/utils.js

Lines 12 to 17 in 64e7dda

	export const getProductFieldObject = ( product, quantity ) => {
	return {
	id: getProductId( product ),
	name: product.name,
	quantity,
	category: getProductCategory( product ),

This leads to `quantity' being incorrect.

Steps to reproduce:

1. Install and enable Google Analytics Debugger.
2. Edit the checkout page to use the Checkout block.
   
3. Open the Console tab in the browser's DevTool.
4. Add items to your cart and go to the Checkout page.
5. Edit the email of the contact information.
6. Inspect the checkout_progress event tracking via the Console tab.

Expected behavior:

The quantity in the event tracking should be the same as each product in the cart.

Actual behavior:

The quantity in the event tracking is the array index value of the products in the cart.