Focusable img elements should fail if they have an empty alt

[WilcoFiers]

I think it would be good if axe-core distinguished between focusable images and non-focusable ones. For example, I think the following should fail:

<img alt="" tabindex="0" />

To do this, we'd need to split up the image-alt rule. Maybe exclude focusable images from it and have those tested in a separate img-alt-focusable rule?