Fix cart item quantity change rollback

[jbruni]

What?

This is a bug fix.

Context: change the quantity of a cart line item by manually editing the quantity input field value.

Currently, when a manual quantity change fails to succeed (for example, because it is greater than the maximum purchasable quantity), the quantity is reverted to zero in the input field.

It should rollback to the previous value. Not to zero.

This PR fixes this bug.

---

It is quite simple to see what is the problem in the code:

• inside the quantity input focus event handler function, this is used as a reference to the event target element (jQuery binds the event target element to the event handler function context);
• as an arrow function is being used for the event handler function, this is not referencing the event target element, as it should, but the enclosing context (Cart class instance)

Thus, the fix was also quite simple: only a matter of using a normal function. This way, we have the event target element properly bound to the function's this context. The preVal (previous value) variable is properly populated with the input's value.

[bigbot]

Autotagging @bigcommerce/storefront-team @davidchin

[karen-white]

Thanks @jbruni ! We'll get this reviewed.

[mattolson]

Thanks @jbruni !

[mattolson]

@jbruni Can you add a changelog entry, and then I'll merge this.