[DowngradePhp81] Support JsonSerializable in AddReturnTypeWillChangeAttributeRector (#380)

* Add JsonSerializable methods to type mapping

* Add test for JsonSerializable

Author: TobiasBg

rules-tests/DowngradePhp81/Rector/ClassMethod/AddReturnTypeWillChangeAttributeRector/Fixture/json_serializable.php.inc

@@ -0,0 +1,26 @@
+<?php
+
+namespace Rector\Tests\DowngradePhp81\Rector\ClassMethod\AddReturnTypeWillChangeAttributeRector\Fixture;
+
+class SomeJsonSerializable implements \JsonSerializable
+{
+    public function jsonSerialize()
+    {
+    }
+}
+
+?>
+-----
+<?php
+
+namespace Rector\Tests\DowngradePhp81\Rector\ClassMethod\AddReturnTypeWillChangeAttributeRector\Fixture;
+
+class SomeJsonSerializable implements \JsonSerializable
+{
+    #[\ReturnTypeWillChange]
+    public function jsonSerialize()
+    {
+    }
+}
+
+?>

rules/DowngradePhp81/Rector/ClassMethod/AddReturnTypeWillChangeAttributeRector.php

@@ -35,6 +35,7 @@ final class AddReturnTypeWillChangeAttributeRector extends AbstractRector
         'Countable' => ['count'],
         'Iterator' => ['current', 'key', 'next', 'rewind', 'valid'],
         'IteratorAggregate' => ['getIterator'],
+        'JsonSerializable' => ['jsonSerialize'],
         'Stringable' => ['__toString'],
     ];