[design-system] icon button accessibility

[macfarlandian]

What needs to be done? Why does it need to be done?
When creating an "icon button" by using the icon prop on a button, there is currently no way to add a text alternative to the icon image for screen readers and other assistive devices. To remedy this we should:

• require a non-empty alt text prop when the icon prop is used (it can be applied to the svg as an aria-label attribute)
• include a role="img" attribute on the SVG to ensure it is recognized by screen readers as "meaningful" rather than "decorative"

Additional context
We worked around this for https://github.com/Recidiviz/recidiviz-dashboards/issues/6119 by putting an Icon inside Button and DropdownTrigger elements instead of passing the icon-related props, and applying these attributes manually