Add core's button CSS classes by default

Problem/Motivation

Similar links found in Drupal core-supplied Views, like the admin/content page, have the appereance of a button, so I suggest we use the already provided CSS classes to improve the styling.

Proposed resolution

This could be made by adding these default CSS classes:
button button--action button--primary

If there isn't a way for a recipe to define that, I suggest we change line 128 of /src/Plugin/views/area/AddContentByBundle.php to
'#default_value' => $this->options['class'] ?: 'button button--action button--primary',

Remaining tasks

• Decide
• Provide a MR