[codemods] Scaffold @stylexjs/codemods package + verification checks (Emotion→StyleX, 1/6)

[ahmedsadid]

What changed / motivation ?

Adds the @stylexjs/codemods package — the foundation for an Emotion → StyleX
migration codemod (#1766). First of ~6 stacked PRs; this one is scaffolding and
test infrastructure only, with no transform logic yet, so it can be reviewed on
its own.

It sets up the package and the three checks every future conversion has to pass:

• the output compiles through @stylexjs/babel-plugin,
• it passes @stylexjs/eslint-plugin at error with no autofixes, and
• its CSS matches the original Emotion output (via @emotion/serialize).

Each check is proven to fail on a deliberately-broken input, not only to pass on
a valid one.

The package is split into a shared engine (src/core/) and per-library adapters
(src/adapters/, Emotion first). core/ never imports from adapters/,
enforced by a test — so a new source library means a new adapter, not an engine
change. The transform itself lands in the next PR.

Linked PR/Issues

Part of #1766

Additional Context

6 test suites, all green. Two end-to-end fixture assertions are skipped until the
transform lands (PR 2). Changes outside the package: a .flowconfig mapping and
an .eslintrc.js ignore for the test fixtures, small flow_modules/ type
stubs, and yarn.lock.

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.

[ahmedsadid]

Prettier CI: failure is a pre-existing upstream line in eslint-plugin/.../cssProperties.js (#1760), untouched by this stack — unrelated to every PR in the series.