Converts Universal Analytics Enhanced Ecommerce products/impressions/promotions dataLayers to the format required by GA4's items array.

1. Refactored code to be more error-resilient.
2. You can now type custom parameter names for the custom dimensions and metrics instead of using the index numbers.
3. Fix item quantity to not send NaN if source is not a number.
4. Fixed item_categoryN to not have the second underscore.
5. Convert quantity to integer to prevent breakage.
6. Prevent error if no ecommerce object found.
7. Added table for mapping custom dimensions and metrics.
8. Initial release.