Update: acknowledge the image role

[scottaohara]

role=image is the preferred default role, over the previous and still valid img role.

update the spec to call out the existence / allowance of the image role.

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labels: needs implementation commitment, needs changelog entry

Closes #481

test case:

<div role=image>foo</div>
<!-- or -->
<div role=image aria-label=bar></div>

• HTML validator
• IBM equal access accessibility checker - already supports the image role
• axe-core
• ARC toolkit

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Preview | Diff

[scottaohara]

though only 1 checker supports this for now, being that it's a role that all browsers support and is the preferred role for how browsers expose the img element - i have high confidence this is fine / checkers will update to account for it.