feat(core): template variants

[tremppu]

as currently it's possible to create templates as in textStyle: 'headline.large' with named property and template name with dot notation, would be great to add variants that could then override styles if needed. This for example would be used for changing fontWeight.

Example is related to: #12

Initial idea for syntax: textStyle: 'headline.large/fw300'

This could be done with base / css + variants setup as in

{
  templates: {
    textStyle: {
      headline: {
        large: {
          css: {
            fontSize: '60px',
            fontWeight: 600,
          },
          variants: {
            fw: {
              300: {
                fontWeight: 300,
              }
            }
          }
        }
      }
    }
  }
}