Support `experimental.swcEnvOptions` config option

[github-actions]

Next.js Change

Next.js added a new experimental.swcEnvOptions config option that exposes SWC's preset-env configuration for automatic polyfill injection (core-js usage/entry mode).

Commit: 2bf38b0
PR: #92272

Config Shape

experimental: {
  swcEnvOptions?: {
    mode?: 'usage' | 'entry'
    coreJs?: string
    skip?: string[]
    include?: string[]
    exclude?: string[]
    shippedProposals?: boolean
    forceAllTransforms?: boolean
    debug?: boolean
    loose?: boolean
  }
}

Impact on vinext

vinext uses Vite for transforms, not SWC's webpack loader, so the polyfill injection behavior itself doesn't directly apply. However:

1. Config validation: If vinext validates next.config.js and rejects unknown keys under experimental, users with this option will get errors. vinext should accept (and ignore) this key.
2. Polyfill parity: Users migrating from Next.js who rely on swcEnvOptions for automatic core-js polyfills will lose that functionality on vinext. We should document this gap or explore a Vite-compatible equivalent (e.g., a Vite plugin for core-js injection based on browserslist targets).

Suggested Action

• Accept experimental.swcEnvOptions in config without erroring
• Emit a warning that polyfill injection is not yet supported in vinext
• Consider future support via a Vite plugin (e.g., vite-plugin-babel with @babel/preset-env or a dedicated polyfill plugin)