feat(theming): provide scoping <div> knock-out for ThemeProvider

[jzempel]

Description

Items of note for this PR:

• Adds support for focusVisibleRef={null} to prevent render of :focus-visible scoping div default
• Adds associated spec tests
• Fills out ThemeProvider prop documentation in prep for website.
• Restores ThemeProvider prop sheet to the example page

Checklist

• 👌 design updates are Garden Designer approved (add the
  designer as a reviewer)
• 🌐 Styleguidist demo is up-to-date (yarn start)
• ⬅️ renders as expected with reversed (RTL) direction
• 🤘 renders as expected with Bedrock CSS (?bedrock)
• ♿ analyzed via axe and evaluated using VoiceOver
• 💂‍♂️ includes new unit tests
• 📝 tested in Chrome, Firefox, Safari, Edge, and IE11

[jzempel]

This repeats the definition from the extended interface. But without it, we aren't able to document the prop. Therefore, I think it's imperative in this one case to make an exception and include it.