Fix generation of prose-headings variant

[thecrypticace]

When using prose-headings:{utility} we were generating .prose-headings\:{utility} h1, h2, h3 which does not work the way we expect. Right now addVariant(…) does not replace multiple uses of & in a selector so we have to use the parallel variant support instead where we pass an array and generate separate rules. For example we have to generate this

.prose-headings\:{utility} h1 { … }
.prose-headings\:{utility} h2 { … }
.prose-headings\:{utility} h3 { … }
…

instead of a single selector.

Fixes #263

[vercel]

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