add discount to amount breakdown (#12)

* add discount to amount breakdown (#11)

* added money and currency value objects

* fix money contract

* fix failed tests

Co-authored-by: abdelwahid elmeghit <abdoo.max@gmail.com>

Author: mhdcodes

composer.json

@@ -27,7 +27,7 @@
     "require-dev": {
         "phpunit/phpunit": "^9.0",
         "squizlabs/php_codesniffer": "^3.4",
-        "phpstan/phpstan": "^0.11.5"
+        "phpstan/phpstan": "^0.12"
     },
     "scripts": {
         "test": "phpunit",