fix: StyledObject type to allow nested selectors

[maximebiloe]

Fix the StyledObject type which was preventing the use of something like

{
  '&:hover': {
    '.sort-arrows': {
      '> svg': {
        color: 'red',
      },
    },
  },
}

because there was no CSSProperties between the &:hover and the .sort-arrows.
Indeed, the type was requiring it.
I got inspired by the former type in https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/styled-components/index.d.ts#L19-L21

[quantizor]

Thank you!