Chart tooltips aren't announced

[thure]

This is related to OfficeDev/microsoft-teams-ui-component-library#142 since the implementation here would be the same.

~This may mean it's worth considering #35 if that might offer a fix out of the box.~

[thure]

Looks like Chart.js v3 doesn't offer any help announcing tooltips either. Chart.js's documentation on accessibility is not at all comprehensive and doesn't mention tooltips at all.