Replace `leadingIcon` and `trailingIcon` with `leadingVisual` and `trailingVisual`

[echarles]

Primer now defines

    /**
     * The leading icon comes before button content
     */
    leadingVisual?: React.ComponentType | null | undefined;
    /**
     * The trailing icon comes after button content
     */
    trailingVisual?: React.ComponentType | null | undefined;