disallow aria-hidden on body element

[scottaohara]

related to https://github.com/w3c/aria/issues/1254

closes #221 - there were comments to do more with aria-hidden rules beyond just disallowing it on body, but i'm going to make a new issue for potentially covering those topics.

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labels: needs implementation commitment, needs changelog entry

• [ ] HTML validator
• [x] IBM equal access accessibility checker
• [x] axe-core already supports this rule
• [x] ARC toolkit

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