Is your feature request related to a problem? Please describe.
Currently the variant prop accepts a string and allows creating variants by extending a single variant. It's not really a problem, but doing so results in bigger theme files compared to a scenario where the variant key could take an array or a string, and then merge all their styles in order to create resulting variant.

Describe the solution you'd like
variant key should be able to accept a string or an array of variants, and then if it's a string - it behaves like how it behaves right now but if it's an array: it should merge the styles of those variants in order that they're mentioned in - sort of like how we extend styles in SCSS. This will result in less number of variants and more crazy combinations.

Describe alternatives you've considered
SCSS does this properly, but I just use Theme UI and then just extend one variant and copy styles from the variants that I want to extend.