Header a11y fix

[xman343]

Changes

#1148 - The only example Main is currently failing. This is due to the lack of an accessible label for the HeaderSplitButton.

• Passing solution: Implemented aria-label "More" for HeaderSplitButton. This helps the screen reader communicate the purpose of the button to the user.

Testing

Testing will be conducted through the Cypress web a11y script. Any breaking changes will be evaluated through unit testing & visual tests. Example also read through NVDA to assess screen reader compatibility.

Docs

TBD

[xman343]

Conflicted on whether to implement the accessible name through VisuallyHidden or a title for the button. I'm going with title for now since it serves the extra function of displaying the label in a popover when it's hovered on. I could change it if we decide that it's unnecessary for this component.