feat: add altText as a data attribute to rendered span

[drizco]

This pull request updates the handling of expandable images to improve accessibility and data consistency. The main change is the addition of a dataAlt attribute, which stores the cleaned alt text without the "expandable" suffix, both in the image processing logic and in the schema.

Expandable image improvements:

• Updated expandableImages logic in src/expandableImages.js to extract and store the alt text without the "expandable" suffix in a new dataAlt property, and to use this cleaned text as the visible value.

Schema updates:

• Modified the schema in utils.js to allow the new dataAlt attribute on span elements representing expandable images.

main repo pr: code-dot-org/code-dot-org#68933

[hannahbergam]

LGTM

[bethanyaconnor]

Is it possible to add a test for this? I don't see an obvious test file for this feature but this repo does have testing set up and it would be good to add. Sorry for adding more work :/

[drizco]

Is it possible to add a test for this? I don't see an obvious test file for this feature but this repo does have testing set up and it would be good to add. Sorry for adding more work :/

of course, will do!

[drizco]

@mgc1194 @stephenliang @Hamms does anyone know if I need to do a version bump here and update the main repo's package json to pull in this change?