[REF] point_of_sale, *pos*: renaming cashier, partner and customer

Multiple refactors
- The module pos_hr had two attributes `cashier` and `employee` referencing to the same actual person. The `employee` attribute was removed for the sake of `cashier`.
- Refactor of the `useSelectEmployee` which returned functions that where used in two components. Instead, a mixin class was implemented and inherited from both components.
- Renaming of the substring `client` and `customer` to `partner` (css included) when it was referencing to the actual `res.partner` object. This way, developers will always know that the object used in the UI is coming from `res.partner`.
- Renaming of the substring `client` to `customer`(css included) when it was referencing to a real life person. This avoid having ambiguity since the term `client` can refers to multiple things (hardware, browser, person, ...)

closes odoo/odoo#83983

Related: odoo/enterprise#24477
Signed-off-by: Masereel Pierre <pim@odoo.com>