add support for new (2025) actualtext, artifact options of graphicx

[xworld21]

graphicx introduced two options at the very end of 2024:

• actualtext is additional text for accessibility (this is meant as 'replacement' text, rather than a descriptive text that goes into alt)
• artifact is a boolean specifying that the image is decorative

Implementation would take only a few lines, but I get the feeling that they will require a schema change and some thinking about the correct HTML equivalent (e.g. if I understand the intention correctly, actualtext should probably hide the fact that <img> is an image by overriding @role and setting @aria-label instead of @alt).