feat: add required checkbox support to Abide

[SassNinja]

Description

This picks up my old PR #8937 whose history got messed up.

Abide did only support required correctly for radio buttons and not for checkboxes. This PR adds checkboxes support.
Now one required checkbox will make the whole group/fieldset required.

• Closes [Abide] Required on Radio Group doesn't work #11683

Types of changes

• Documentation
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (anything that would change an existing functionality)
• Maintenance (refactor, code cleaning, development tools...)

Checklist

• I have read and follow the CONTRIBUTING.md document.
• The pull request title and template are correctly filled.
• The pull request targets the right branch (develop or develop-v...).
• My commits are correctly titled and contain all relevant information.
• I have updated the documentation accordingly to my changes (if relevant).
• I have added tests to cover my changes (if relevant).

[joeworkman]

This pull request has been mentioned on Foundation Open Source Community. There might be relevant details there:

https://foundation.discourse.group/t/foundation-for-sites-v6-6-0-is-here-farout/30/1