Divider: Use span element
As the Divider component is not used as a container element, we now use a span. This allows it to be used inside button elements, such as a Tab, without producing invalid html.
Divider: Rename 'standard' weight to 'regular'.
Divider: Add strong weight variant, e.g. <Divider weight="strong">.
Note that this also affects the dividers prop on both Stack and Tiles, e.g. <Stack space="medium" dividers="strong">. You can still pass a boolean prop if you want to render the default divider styling, e.g. <Stack space="medium" dividers>, so this change is backwards compatible.