Expose storage entity as a top-level twig variable

Problem/Motivation

When theming a Storage entity in twig, I can access the entity itself like this:

{{ elements['#storage'] }}

Most core entities go a step further and add their entity to the first level of Twig variables, so they can be accessed like this:

{{ node }}
{{ user }}

Proposed resolution

Do the same thing for storage entities, for consistency with Drupal core and better themer experience.