Allow carousel's scrollable content to be tab-able with visual focus

[corinagum]

Fixes #3814
Fixes #3835

Changelog Entry

• Fixes #3814. Allow carousel's scrollable content to be tabbable, by @corinagum in PR #3841
• Fixes #3834. Ensure carousel attachments are read by AT on tab focus, by @corinagum in PR #3841

Description

Carousel focus on children needed to be added in order to allow tabbing and AT to read contents.

Focus style will match from the following variables:
transcriptVisualKeyboardIndicatorColor
transcriptVisualKeyboardIndicatorStyle
transcriptVisualKeyboardIndicatorWidth

Specific Changes

#3814 - Allow carousel's scrollable content to be tab-able with visual focus

• Add tabindex to carousel card(s)
• Add focus indicator to carousel's children

#3835 - Ensure carousel attachments are read by AT on focus

• Fixed by 3814; No test because we don't have a way to verify if AT reads content or not

[Other]

• update CODEOWNERS
• eslint comments in Playground
• Remove unused asset from focusManagement.(...).html test

• I have added tests and executed them locally
• I have updated CHANGELOG.md
• I have updated documentation

Review Checklist

This section is for contributors to review your work.

• Accessibility reviewed (tab order, content readability, alt text, color contrast)
• Browser and platform compatibilities reviewed
• CSS styles reviewed (minimal rules, no z-index)
• Documents reviewed (docs, samples, live demo)
• Internationalization reviewed (strings, unit formatting)
• package.json and package-lock.json reviewed
• Security reviewed (no data URIs, check for nonce leak)
• Tests reviewed (coverage, legitimacy)

[corinagum]

This won't be merge-able until #3836 gets in due to MB changes

3836 is merged

[compulim]

Need some work to make it conform with CSS BEM.

[compulim]

Looks great!