Added GitHub Actions Slack notification.

[martnpaneq]

🚀 Summary

Added ckeditor5-dev-ci-notify-github-actions-status, a GitHub Actions equivalent of ckeditor5-dev-ci-notify-circle-status that posts a Slack notification summarizing a failed workflow run (commit author, branch, run URL, duration). Intended to be invoked from a step or job gated by if: failure().

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📌 Related issues

• See ckeditor/ckeditor5-internal#4450

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💡 Additional information

Optional: Notes on decisions, edge cases, or anything helpful for reviewers.

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