Layouts have support for per-region attributes

All layout templates now have a variable called region_attributes available.
Similar to the existing variable content, this is an array keyed by region name.
Inside are the attributes for that HTML tag.

Before

// In mylayout.html.twig
<div{{ attributes }}>
  <div class="region-first">
    {{ content.first }}
  </div>
  <div class="region-second">
    {{ content.second }}
  </div>
</div>

After, Option 1

// In .theme
function template_preprocess_mylayout(&$variables) {
  $variables['region_attributes']['first']['class'][] = 'region-first';
  $variables['region_attributes']['second']['class'][] = 'region-second';
}

// In mylayout.html.twig
<div{{ attributes }}>
  <div {{ region_attributes.first }}>
    {{ content.first }}
  </div>
  <div {{ region_attributes.second }}>
    {{ content.second }}
  </div>
</div>

After, Option 2

// In mylayout.html.twig
<div{{ attributes }}>
  <div {{ region_attributes.first.addClass('region-first') }}>
    {{ content.first }}
  </div>
  <div {{ region_attributes.second.addClass('region-second') }}>
    {{ content.second }}
  </div>
</div>