fix: invalid @ rule body when using dynamic styles in pseudo elements

[skovhus]

What changed / motivation ?

Fix runtime error Invalid @ rule body when using dynamic styles in pseudo elements, such as:

const styles = stylex.create({
  repro: (var: string) => ({
    '::after': {
      color: var, // FAILS
    },
  }),
});

Linked PR/Issues

Fixes #1396

Additional Context

Steps to reproduce

1. In this repository, use the examples/example-vite-react project.
2. Add the following style:

const styles = stylex.create({
  repro: (color: string) => ({
    '::after': {
      color,
    },
  }),
});

Pre-flight checklist

• I have read the contributing guidelines
  Contribution Guidelines
• Performed a self-review of my code

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