AttributeHelper is introduced to manage mixed array/Attribute object operations

Mixed usage of arrays and Attribute objects in rendering causes issues due to the fact that Array objects are not correctly working with all array_* functions and NestedArray methods.

In order to manage properly checking the existence of an attribute, and merging attributes a new Drupal\Core\Template\AttributeHelper class is introduced, with two static methods:

• ::attributeExists
• ::mergeCollections

In addition, two methods are added to the Drupal\Core\Template\Attribute class, that allow to manage the case when it's definite that we are dealing with Attribute objects:

• ::hasAttribute
• ::merge

Themers can leverage on the AttributeHelper methods instead of array functions or NestedArray, without having to predetermine the type of the argument,

for checking existence of an attribute:

before:

      if (array_key_exists($key, $variables['attributes'])) {
        continue;
      }

after

      if (AttributeHelper::attributeExists($key, $variables['attributes'])) {
        continue;
      }

for merging two collections of attributes:

before:

      $variables['attributes'] = NestedArray::mergeDeep($variables['attributes'], $variables[$key]['#attributes']);

after

      $variables['attributes'] = AttributeHelper::mergeCollections($variables['attributes'], $variables[$key]['#attributes']);