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/Invoice.php

@@ -521,6 +521,7 @@ class Invoice extends Model implements HasMedia
 
     public static function createTaxes($invoice, $taxes)
     {
+
         $exchange_rate = $invoice->exchange_rate;
 
         foreach ($taxes as $tax) {