Fix/vertical slider handle negative data start

[altras]

Description

In this PR we propose a fix for Vertical Slider to handle negative data-start values. The change is isolated in _value and _setHandlePos.

I appreciate your feedback 🙇

motivation

Having a negative data-start value extends the use cases for the Slider.

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).

[kball]

I have tested this in isolation, as well as reviewed the code. Merging.

[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