LibWeb: Code cleanup for attribute getting in Node::name_or_description

[sideshowbarker]

This change includes two commits:

• 5e65b07c37d LibWeb: Minor code cleanup; use HTML::AttributeNames::value, not string
• e9f8b2d4265 LibWeb: Use ARIA::AriaData rather than element.has_attribute

I don’t know that the ARIA::AriaData change is any better for performance — but it seems better for maintainability at least. And since we have this ARIA::AriaData abstraction in the code, it seems preferable to use it consistently when working with code related to ARIA requirements.