Create new rule for image button name descriptive

[EmmaJP]

New PR to replace #1184. This only adds the new rule to check that the accessible name of an image button is descriptive, and does not change any existing rules. As decided in discussions about issue #1049.

Closes issue(s):

• closes and replaces #1184
• potentially closes #1049

Need for Final Call: This will require a 2 weeks Final Call, as it is a new rule.

---

Pull Request Etiquette

When creating PR:

• [ ] Make sure you're requesting to pull a branch (right side) to the develop branch (left side).
• [ ] Make sure you do not remove the "How to Review and Approve" section in your pull request description

After creating PR:

• [ ] Add yourself (and co-authors) as "Assignees" for PR.
• [ ] Add label to indicate if it's a Rule, Definition or Chore.
• [ ] Link the PR to any issue it solves. This will be done automatically by referencing the issue at the top of this comment in the indicated place.
• [ ] Optionally request feedback from anyone in particular by assigning them as "Reviewers".

When merging a PR:

• [ ] Close any issue that the PR resolves. This will happen automatically upon merging if the PR was correctly linked to the issue, e.g. by referencing the issue at the top of this comment.

How to Review And Approve

• Go to the “Files changed” tab
• Here you will have the option to leave comments on different lines.
• Once the review is completed, find the “Review changes” button in the top right, select “Approve” (if you are really confident in the rule) or "Request changes" and click “Submit review”.
• Make sure to also review the proposed Final Call period. In case of disagreement, the longer period wins.