feat: Support multiple classes per theme in withThemeByClassName

[damianstasik]

In my storybook I wanted to add different Tailwind classes to parentElement depending on a theme. For example, dark theme would have text-white bg-black and white theme: text-black bg-white. I was surprised to see the classic The token can not contain whitespace exception. This PR added support for multiple classes per theme:

withThemeByClassName({
  themes: {
    dark: 'text-white bg-black',
    white: 'text-black bg-white`,
  },
  defaultTheme: 'dark',
}),

[ShaunEvening]

Hey @damianstasik

Thank you so much for contributing to this new addon! I really appreciate it 🙏