Adding default a11y to useSortBy hook

[BillyLevin]

At the moment, the useSortBy hook doesn't come with much default accessibility (as far as I'm aware). I think it would be a good idea to add some sensible defaults to the getSortByToggleProps function. For example:

• tabindex on the sort element
• using aria-label="Toggle Sort" in addition to the title="Toggle Sort" attribute already being used
• role="button" on the sort element
• keydown listener to toggle sorting when user presses Enter or Space key

Another one is aria-sort, however it looks like that should only be used directly on a th element, so if the developer is applying these props to an element nested inside, it may not be correct. Not sure what the best way would be to incorporate this.

If this is something you'd like included, or if anyone has any suggestions/corrections (I'm no a11y expert) please feel free to respond below :)

Thanks!