Toggle: Improve screen-reader accessibility

[c-w]

Pull request checklist

• Addresses an existing issue
• Include a change request file if publishing
• New feature, bugfix, or enhancement
  • Includes tests
• Documentation update

Description of changes

The Toggle is currently not very accessible for screen-reader users as it doesn't have hooks that let us provide context on the option that's being toggled. This pull request adds two optional aria-labels that lets us provide more context on the on/off states for screen-reader users.

Video of screen-reader behavior after applying the pull-request.

It's not ideal that the screen-reader re-announces the toggle onText and offText, however, this is not easily preventable as many browser/screenreader combinations will announce aria-hidden children while reading the parent.

Focus areas to test

1. Open Narrator on Windows 10.
2. Open the Toggle example in Edge.
3. Navigate to "enabled and checked" toggle.
4. Hear the screen-reader speak the onAriaLabel.
5. Press enter to state-switch the toggle.
6. Hear the screen-reader speak the offAriaLabel.

[c-w]

@mikewheaton: Thanks for the quick sign off. I don't have merge-access, so could you please also hit merge before master advances again?