Add support for new form type image-description

[tiritea]

Closes ##759
Closes ##760

Why is this the best possible solution? Were any other approaches considered?

This solution and others considered were discussed extensively on the ODK Forum. See https://forum.getodk.org/t/placeholder-alt-text-for-images-eg-screen-readers/50868

What are the regression risks?

This adds new new (optional) translatable form type, to the existing image, big-image, audio and video media types. It should not impact the existing functionality of these existing media types. XForm clients that do not support or recognize the new image-description form type can safely ignore it with no impact on existing XForm functionality.

This PR also adds a new testcase for a choice option with a big-image specified but without a corresponding image, because it is almost identical to the check now needed for image-description. This error condition is currently not being checked, and results in an invalid XForm being generated [note, this image prerequisite is being checked in the main survey, but not for choice options. It was probably overlooked]. The new big-image check will mean previously invalid XLSForms with this inconsistency will now fail to be processed, but I believe this is actually the desired behavior.

Does this change require updates to documentation? If so, please file an issue here and include the link below.

Yes, this new ODK form type will need to be documented in the XLSForm Docs.
See XLSForm/xlsform.github.io#279

Before submitting this PR, please make sure you have:

• included test cases for core behavior and edge cases in tests
• run python -m unittest and verified all tests pass
• run ruff format pyxform tests and ruff check pyxform tests to lint code
• verified that any code or assets from external sources are properly credited in comments - Not Applicable

Other important notes:

This is my first pyxform PR; please be gentle... :-)
Attn: @RuthShryock