feat(Dropdown): add a11y prop to tab moves focus

[TheSharpieOne]

This adds a new a11y prop to Dropdrop, defaults to true.
When the a11y prop is explicitly set to false:
This changes the way tab works within the Dropdown.
tab/shift-tab will move focus to the next logic item:
tab will move down until the end. Then, at the end of the menu it will focus the next focusable item on the page and close the menu.
shift-tab will move up until the start. Then, at the beginning of the menu it will focus the toggle and close the menu.

Fixes #1441

• Bug fix
• New feature
• Chore
• Breaking change
• There is an open issue which this change addresses
• I have read the CONTRIBUTING document.
• My commits follow the Git Commit Guidelines
• My code follows the code style of this project.
• My change requires a change to the documentation.
• • I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[rhdtl78]

I faced the warning with a11y attribute on dropdown.

Warning: Received true for a non-boolean attribute a11y.

If you want to write it to the DOM, pass a string instead: a11y="true" or a11y={value.toString()}.
in div (created by Dropdown)
in Manager (created by Dropdown)
in Dropdown ...