Added declare strict types to all files

src/Serializer/BigNumberNormalizer.php

@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /*
  * This file is part of Part-DB (https://github.com/Part-DB/Part-DB-symfony).
  *
@@ -17,7 +20,6 @@
  *  You should have received a copy of the GNU Affero General Public License
  *  along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
-
 namespace App\Serializer;
 
 use Brick\Math\BigNumber;
@@ -49,4 +51,4 @@ class BigNumberNormalizer implements NormalizerInterface
             BigNumber::class => true,
         ];
     }
-}
+}