Fix: Add support for Navigation Button API (fixes #90 #92)

[swashbuck]

Fixes #90 #92

Fix

Related to #90

• Integration with Navigation Button API which allows for custom nav button labels
• Switch nav button template to JSX
• Update example.json

Related to #92

• Prefix .visua11y-btn class with .nav__ e.g .nav__visua11y-btn
• Apply aria-hidden="true" to the .icon
• Replace the tooltip with <button> level aria-label.

Testing

Add config to course.json:

"_extensions": {
  "_visua11y": {
    "_navOrder": 100,
    "navLabel": "Accessibility",
    "_navTooltip": {
      "_isEnabled": true,
      "text": "Visual accessibility settings"
    }
  }
}

[swashbuck]

Thanks, @kirsty-hames .

@oliverfoster Do you see anything that needs to be changed? I poached most of the code from https://github.com/adaptlearning/adapt-contrib-pageLevelProgress/pull/174

[github-actions[bot]]

:tada: This PR is included in version 2.4.1 :tada:

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