Make img alt information available

[barnabywalters]

Currently, when parsing mf2 data which includes alt information for properties parsed from images, XRay has no way of making the alt information available to library consumers.

This might additionally be a JF2 issue, as the current spec doesn’t mention image alt values at all https://jf2.spec.indieweb.org/

Potential solutions which come to mind:

• Provide img+alt structures where alt information is available, forcing consumers to check whether every image-based property is a string or img+alt array before consuming (breaks existing code)
• Provide img+alt structures by default on every property which is parsed as an image, with empty alt values if none is found. Breaks existing code but is more consistent.
• Use either the ref(erences) list or a new “alts” list on jf2 objects to provide the alt value, indexed by the image URL. This would not break any existing code afaik