feat(button): use full corner rounding on icon only variant

The icon only variant will always be fully rounded and does not need to
use the calculated value that accounts for wrapping text.

components/button/index.css

@@ -42,6 +42,10 @@ governing permissions and limitations under the License.
   --mod-progress-circle-position: absolute;
   /* stylelint-disable-next-line spectrum-tools/no-unknown-custom-properties */
   --spectrum-downstate-perspective: max(var(--spectrum-downstate-height), var(--spectrum-downstate-width) * var(--spectrum-component-size-width-ratio-down));
+
+  &.spectrum-Button--iconOnly {
+    --spectrum-button-border-radius: var(--spectrum-corner-radius-full);
+  }
 }
 
 .spectrum-Button--sizeS {