Revert "Fully revert NodeList changes"

This reverts commit 75d647a.

src/Language/Visitor.php

@@ -224,7 +224,7 @@ public static function visit(object $root, array $visitor, ?array $keyMap = null
                                 throw new \Exception("Can only add Node to NodeList, got: {$notNode}.");
                             }
 
-                            $node[$editKey] = $editValue;
+                            $node->set($editKey, $editValue);
                         } else {
                             $node->{$editKey} = $editValue;
                         }
@@ -246,7 +246,7 @@ public static function visit(object $root, array $visitor, ?array $keyMap = null
                         ? $index
                         : $keys[$index];
                     $node = $parent instanceof NodeList
-                        ? $parent[$key]
+                        ? $parent->get($key)
                         : $parent->{$key};
                 }
                 if ($node === null) {

tests/Error/ErrorTest.php

@@ -114,8 +114,8 @@ public function testSerializesToIncludeMessageAndLocations(): void
     {
         $ast = Parser::parse('{ field }');
         /** @var OperationDefinitionNode $operationDefinition */
-        $operationDefinition = $ast->definitions[0];
-        $node = $operationDefinition->selectionSet->selections[0];
+        $operationDefinition = $ast->definitions->get(0);
+        $node = $operationDefinition->selectionSet->selections->get(0);
         $e = new Error('msg', [$node]);
 
         self::assertEquals(