feat(accordion): add css custom properties

[yuhsianw]

PR Checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• If applicable, have a visual design approval

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• clarity.design website / infrastructure changes
• Other... Please describe:

What is the current behavior?

Issue Number: #278

What is the new behavior?

--box-shadow, --icon-visibility, and --icon-margin CSS custom properties added to cds-accordion-header.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[github-actions]

👋 @yuhsianw,

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• 🎉 The build for this PR has succeeded
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Thank you,

🤖 Clarity Release Bot

[williamernest]

I left a comment regarding initializing the new values and adding a comment above the :root block to elaborate on why these fields are present.

[github-actions]

🎉 This PR is included in version 6.9.0 🎉

The release is available on:

• GitHub release
• v6.9.0

Your semantic-release bot 📦🚀

[github-actions]

Hi there 👋, this is an automated message. To help Clarity keep track of discussions, we automatically lock closed PRs after 14 days. Please look for another open issue or open a new issue with updated details and reference this one as necessary.