Add role="image" as synonym for role="img" #758

https://www.w3.org/Bugs/Public/show_bug.cgi?id=28934

With the exception of "combobox" and "contentinfo", all existing ARIA roles are complete words or concatenations of complete words. In almost all cases, the role's name is the same as the complete word or term by which the element is conventionally referred to, e.g. link, article, button, list, listitem, combobox. The "img" role is the exception, and authors are prone to mistakenly, but understandably, use role="image", which won't work.

Recommend adding role="image" as a synonym for role="img" to avoid author confusion and error.