Implement the IconButton component from the Probo Radix UI Figma: a
square, icon-only <button> sharing Button's size 1-4 and
variant/color/highContrast matrix, with a loading spinner state. The
single icon is passed as children; callers provide an aria-label.
Corner radius is bound to size. Includes a paired IconButtonSkeleton.
Signed-off-by: Émile Ré <emile@probo.com>
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