[unplugin] Preserve Vite plugin return types

[smorimoto]

What changed / motivation ?

The Vite adapter declares its return value as any, so adding it to a Vite configuration loses plugin type information and triggers typescript/no-unsafe-assignment in projects using type-aware linting.

Use the VitePlugin type exported by the existing unplugin peer dependency, and share the Vite factory declaration with stylex.vite() and the named unplugin export. The runtime behaviour and optional configuration argument remain unchanged.

Linked PR/Issues

N/A

Additional Context

This change only updates the two Vite-related declarations.

Validation:

• yarn test passed (build, Flow, package tests, TypeScript, Prettier and ESLint).
• Existing TypeScript tests and targeted Prettier checks passed for the final diff.

Pre-flight checklist

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

[vercel]

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

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[Copilot]

🟡 Changes recommended

The updated index.d.ts uses typeof on a type-only import (import type vite ...), which will fail TypeScript type-checking.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR updates @stylexjs/unplugin’s TypeScript declarations to preserve the Vite plugin return type (instead of any), and adds a TypeScript regression fixture to ensure the factory return types remain non-any and compatible with unplugin’s VitePlugin.

Changes:

• Type @stylexjs/unplugin/vite’s factory as returning VitePlugin instead of any.
• Reuse the Vite factory type for stylex.vite() / unplugin.vite() in the root index.d.ts.
• Add a dedicated TypeScript test project + fixture to assert non-any return types and VitePlugin assignability.

File summaries

File	Description
yarn.lock	Updates lock entry to include unplugin@^2.3.11 selector.
packages/typescript-tests/unplugin/vite.ts	New TS fixture asserting Vite factory return types are not any and satisfy VitePlugin.
packages/typescript-tests/tsconfig.unplugin.json	New TS config enabling skipLibCheck for the unplugin-focused type assertions.
packages/typescript-tests/package.json	Runs the new unplugin TS project and adds needed dev deps.
packages/@stylexjs/unplugin/src/vite.d.ts	Changes Vite factory return type from any to VitePlugin.
packages/@stylexjs/unplugin/src/index.d.ts	Shares the Vite factory type on stylex.vite via typeof.

Review details

• Files reviewed: 3/6 changed files
• Comments generated: 0
• Review effort level: Lite

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