[FIX] point_of_sale: fp map 2m taxes // align with account + Tests

Before this commit, it was not possible to map one tax to multiple taxes
in the point of sale, just as it is in the account module.

This commit implements fiscal positions that can map to various taxes
on POS.

Furthermore, id adds test to secure that somewhat implicit feature on which
several localisations seem to depend upon.

A typical use case if one tax is not replaced but complemented by another.

**Description of the issue/feature this PR addresses:**
- Taxes can only be replaced in POS.
- Not complemented by a peer tax.
- Behavior not aligned with backend fp mapping.

**Current behavior before PR:**
- Tax cannot be mapped to several taxes (aka: complemented: "to itself and other") on POS

**Desired behavior after PR is merged:**
- Tax can be mapped (aka: complemented) to various taxes on POS

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@xoe-labs

closes odoo/odoo#26415

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