proposal: storybook - add select box option for variants

[joerdav]

I'm working on a component library for tailwindcss, but I think there's some missing functionaility in the storybook package.

I plan on adding these once templ is upgraded to 1.18 as I think the solution will be different with generics.

Most component libraries have a concept of variants, so having a dropdown for these would be extremely valuable.