Issue with tracking quantity changes in cart

[martynmjones]

Describe the bug:

At the moment we're tracking changes to product quantities in the cart (when using blocks) with a change_cart_quantity event. This isn't an event that's supported by UA or GA4.

It could be used as a custom event but additional reporting would need to be set up in Google Analytics for that and there's no reference to it in the documentation so I'm unsure of its original purpose.

From what I can gather, it appears that the correct way to register the events would be to send add_to_cart or remove_from_cart events with the number of items removed.

Unfortunately, the current (experimental) -cart-set-item-quantity action gives us the total number of a product in the cart but not the difference. We'll likely either need to open a PR in blocks to change that or rely on the cart state instead but it will require further exploration.