Add max_files setting

Add the ability to limit the number of files that can be added to the queue.

      $form['upload'] = [
        '#type' => 'plupload',
        '#upload_validators' => [
          'file_validate_extensions' => ['jpg png'],
        ],
        '#plupload_settings' => [
          'runtimes'      => 'html5',
          'chunk_size'    => '1mb',
          'unique_names'  => TRUE,
          'max_files' => 1,
        ],
      ];

Based on solution [Link] from FileField Sources Plupload [ Link → ]