[codemods] Convert conditional css (:hover, :focus, media) to StyleX (Emotion→StyleX, 3/6)

[ahmedsadid]

What changed / motivation ?

Part of migrating Emotion styles to StyleX (#1766). The previous PR (#1769) did
flat static styles; this adds the conditional ones — pseudo-classes,
pseudo-elements, and media queries — which StyleX writes as condition-keyed
values.

// before
<input css={{
  color: 'black',
  ':focus': { color: 'blue' },
  '@media (min-width: 600px)': { fontSize: 18 },
}} />

// after
const styles = stylex.create({
  field: {
    color: { default: 'black', ':focus': 'blue' },
    fontSize: { default: null, '@media (min-width: 600px)': 18 },
  },
});
<input {...stylex.props(styles.field)} />

• Handles :hover, :focus, pseudo-elements (::before/::after), and media
  queries. :hover also gets StyleX's @media (hover: hover) guard.
• Emotion resolves overlapping conditions by source order, StyleX by fixed
  priority. When they'd pick different winners (e.g. :focus vs :hover), the
  file is refused rather than converted with different behavior.
• Still refuses what it can't handle safely: cross-element selectors (& > li),
  plus everything the previous PR skipped (dynamic values, shorthands, template
  literals).

Testing

• Before/after fixtures for :hover, :focus + media, and ::before — each
  result compiles, passes lint, and renders the same CSS as the Emotion input.
• Two "leave it unchanged" fixtures: one where Emotion and StyleX would disagree
  on the winner, one with a selector targeting a child element. Both confirm the
  file is left untouched.
• Unit tests that run the priority-conflict check and the StyleX code generator
  on plain style objects.

Linked PR/Issues

Part of #1766 · builds on #1769

Additional Context

Stacked on #1769, but stands on its own to review.

Pre-flight checklist

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

[vercel]

@ahmedsadid is attempting to deploy a commit to the Meta Open Source Team on Vercel.

A member of the Team first needs to authorize it.