[ErrorHandler] Add return type hints

Added return type hints for normalize & supportsNormalization (v6.0)

Author: mohamedGasmii

controller/error_pages.rst

@@ -220,7 +220,7 @@ contents, create a new Normalizer that supports the ``FlattenException`` input::
 
     class MyCustomProblemNormalizer implements NormalizerInterface
     {
-        public function normalize($exception, string $format = null, array $context = [])
+        public function normalize($exception, string $format = null, array $context = []): array
         {
             return [
                 'content' => 'This is my custom problem normalizer.',
@@ -231,7 +231,7 @@ contents, create a new Normalizer that supports the ``FlattenException`` input::
             ];
         }
 
-        public function supportsNormalization($data, string $format = null)
+        public function supportsNormalization($data, string $format = null): bool
         {
             return $data instanceof FlattenException;
         }