Allowed passing @tagName for dynamic tag

I made 3 updates in this (very first) feature release. 🎉

They facilitate accessibility and semantic HTML when you make container queries in your app or addon.

• Added ember-element-helper as a dependency to allow passing @tagName to create a tag other than <div>. To be safe, @tagName is considered immutable in ember-container-query. Once you set the value, changing it will not update the tag.
• Expanded the role of ...attributes in the <ContainerQuery> component. In addition to attributes for style, you may pass attributes related to accessibility, such as ARIA attributes and roles.
• Installed ember-a11y-testing as a dev-dependency for internal testing.

Much thanks to @chadian, @MelSumner, and @hergaiety for providing feedback and help in implementing dynamic tag. 🧡