Add per-region attributes using region_attributes variable

Layouts all have a variable called region_attributes that core uses to assign attributes to regions, as per this change record: https://www.drupal.org/node/2886860

It looks like display suite ignores this variable and does it's own thing in ds_preprocess_ds_layout.

Lets make use of region_attributes to keep things consistent with core.

Example for a custom DS based layout with regions region_1 and region_2

Before

<div{{ attributes }}>
  <div{{ region1_attributes }}>
    {{ region_1 }}
  </div>
  <div{{ region2_attributes.addClass('foo') }}>
    {{ region_2 }}
  </div>
</div>

After

<div{{ attributes }}>
  <div{{ region_attributes.region_1 }}>
    {{ region_1 }}
  </div>
  <div{{ region_attributes.region_2.addClass('foo') }}>
    {{ region_2 }}
  </div>
</div>