Improve handling Product Page Variant Select Quantities

[chrismekim]

In add-to-cart-container.liquid, we have a quantity select dropdown and a variant select dropdown.

For lack of time, the quantity select dropdown was implemented to show options 1 - 10, regardless of the selected product variant's available inventory. This was discussed in this PR.

The problem:

• A user can try to add more items than available. They won't be able to successfully do this, but there is no feedback that this action failed.

Possible solutions:

• Have eventListeners to listen when the "selected" variant changes, and update the quantity based on the selected product variant's inventory
• Show a notification to let the user know they are not able to add the quantity selected.