[IMP] point_of_sale: Split negative and positive anglo saxon lines

A single anglo saxon line was created in the pos.session journal
entries, even if some products were returned. We now distinguish
positive from negative lines.

E.g. Selling 10 Product1 (cost=10) and returning 5 Product1 used to
create the following line:
```
Stock Interim Account (Delivered)          0      50
```
and now creates the following lines:
```
Stock Interim Account (Delivered)          0     100
Stock Interim Account (Delivered)          50     0
```

Task ID: 2126521

closes odoo/odoo#55086

Signed-off-by: pimodoo <pimodoo@users.noreply.github.com>

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