Split/Stack layouts - add support for component prop on items

I noticed <StackItem /> and <SplitItem /> always render as a <div> but their parents (<Stack /> and <Split />) allow for a component prop that lets you specify what element to use. If you do something like <Stack component="span"><StackItem>, you'll get <span><div>, which is invalid HTML. We should allow for customization on the element rendered as the split/stack items.