fix(root): implemented icon variant for back to top component

[MI6-824]

Added functionality that changes the variant of the back-to-top component based on the size of the device screen

Checklist

• [ ] I have manually accessibility tested any changes, if relevant.
• [X] I have raised this pull request against the develop branch

[github-actions[bot]]

View your branch deployment here: https://mi6.github.io/ic-design-system-githubpages/branches/959-guidance-site-implement-icon-only-variant-of-back-to-top-for-smaller-viewports

[gd2910]

PR now stale.