[Vouchers] Support mixed tax discounted by vouchers

[mkllnk]

Description

The voucher tax logic assumes that tax is either included in price or excluded from price across all line items of the discounted order. But multiple tax rates can apply to the same product and therefore one line item can have included and excluded tax. I even heard stories of that applying in real life in some countries. 🙃

• Original spec: [Vouchers] Implement bare minimum checkout #10432 (comment)
• Issue stated: 10432 vouchers bare minimum checkout #10587 (comment)

Expected Behavior

If some line items have included and excluded tax then the applied voucher also has included and excluded tax respectively.

Actual Behaviour

If some line items have excluded tax then the voucher only looks as excluded tax and doesn't contain included tax.

Steps to Reproduce

1. Create two tax rates, one included in price and one additional.
2. Apply both tax rates to some products and some fees.
3. Add items with and without tax to an order.
4. Apply a voucher.
5. Verify the calculations. Included and excluded tax should be displayed.

Animated Gif/Screenshot

Workaround

Severity

On a bug level it's only an s5 but this is also tech debt. I think that resolving this issue will make vouchers more robust and it will make testing easier with less confusion.

Your Environment

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Possible Fix