[html-aam] Add img w/controls attr mapping

[rahimabdi]

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🔄 this PR updates the following sspecs:

• html-aam preview — html-aam diff

Closes w3c/html-aam#601

This PR makes two changes:

• Adds new mapping table for img element with a controls attribute, with a callout that empty alt should be ignored
• Adds img element to controls attribute mapping (along with existing audio and video)

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[scottaohara]

@rahimabdi thanks again for getting this going.

from what we talked about, i think we still need to separate out the attribute mapping for the img[controls] from the other instances of [controls]. because this still needs to call out that the controls attr on an img needs to sprout a parent role=group in the a11y tree to contain the img and any controls that will be rendered as a sibling to the img

i.e., the img[controls] attribute mapping table should mimic the video element mappings, but specifically mention the mappings don't alter the img, but instead add a parent group / sibling toolbar of controls.

then the comments for img w/controls and the img[controls] attribute tables can reference each other in the spec. simply "see element" / "see attribute"

[scottaohara]

see prior comment