feat(uui-button): add transition property

[julczka]

Description

This PR adds a --uui-button-transition custom property to #button selector so you can set a transition on the button properties if you want to (closes #764 )
It also cleans up some unused code and adds styling for :focus-visible pseudoclass ( closes #749 ). See image below.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Chore (minor updates related to the tooling or maintenance of the repository, does not impact compiled assets)

Motivation and context

How to test?

Screenshots (if appropriate)

Checklist

• If my change requires a change to the documentation, I have updated the documentation in this pull request.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

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[github-actions]

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