Headings element modifiers not working in legacy mode

[tylerhunt]

What version of @tailwindcss/typography are you using?

HEAD

What version of Node.js are you using?

v17.4.0

What browser are you using?

Safari

What operating system are you using?

macOS

Reproduction repository

https://github.com/tylerhunt/tailwind-bugs/tree/legacy-element-modifiers-headings

Describe your issue

When using an element modifier for headings like prose-headings:text-red-500, only the h1 elements have the specified utility class applied.

The variant that's added has a class of .prose-headings\:text-red-500 h1, h2, h3, h4, th, since the addVariant call is using & ${selector}, but for headings the value of selector is h1, h2, h3, h4, th.