Adding Flat Tax support with fixed amount (#253)

* Possibility to set a fixed amount on tax types settings

* Pint and manage flat taxes on items

* Fix display errors and handle global taxes

* Tests

* Pint with PHP 8.2 cause with PHP 8.3 version it cause workflow error

* Merging percent and fixed amount into one column

* Now display the currency on SelectTaxPopup on fixed taxes

app/Models/Tax.php

@@ -21,6 +21,7 @@ class Tax extends Model
         return [
             'amount' => 'integer',
             'percent' => 'float',
+            'fixed_amount' => 'integer',
         ];
     }