support next build --webpack flag

[leoortizz]

Description

This PR adds support for the next build --webpack command in the Next.js framework integration. With Next.js 16 defaulting to Turbopack, this change allows users to explicitly opt-out and force Webpack usage by defining "build": "next build --webpack" in their package.json.

Previously, firebase deploy would ignore this flag and warn about a custom build script. This update detects the flag, forwards it to the build process, and suppresses the warning for this specific valid configuration.

Fixes #9749

Scenarios Tested

• Standard Build: Verified that "build": "next build" continues to work as expected without warnings.
• Webpack Opt-in: Verified that "build": "next build --webpack" correctly forwards the --webpack flag to the build command and suppresses the custom script warning.
• Custom Scripts: Verified that unsupported custom scripts (e.g., "build": "next build --verbose") still trigger the standard warning and do not forward arbitrary flags.
• Unit Tests: Added unit tests for the new getBuildScript utility and verified existing tests pass.

Sample Commands

Users can now use the following in their package.json:

{
  "scripts": {
     "build": "next build --webpack"
   }
}

[gemini-code-assist]

Summary of Changes

Hello @leoortizz, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly improves the Firebase CLI's handling of Next.js projects by adding explicit support for the next build --webpack command. This change ensures that users who wish to use Webpack for their Next.js builds, particularly in light of Next.js 16's default to Turbopack, can do so without encountering warnings or unexpected behavior from the CLI. The underlying implementation involves a new utility for build script detection and a refactoring of the custom build script warning mechanism, making the framework integrations more robust and user-friendly.

Highlights

• Next.js Webpack Flag Support: The Firebase CLI's Next.js framework integration now officially supports the next build --webpack command, allowing users to explicitly opt for Webpack instead of the default Turbopack.
• Warning Suppression: The CLI will no longer issue a warning for next build --webpack as a custom build script, recognizing it as a valid configuration.
• Build Script Detection Utility: A new utility function, getBuildScript, has been introduced to reliably extract the build script from package.json.
• Refactored Warning Logic: The warnIfCustomBuildScript function has been refactored to accept the build script directly, improving its reusability and testability across different frameworks.
• Framework-wide Integration: The new getBuildScript utility and refactored warning logic have been integrated into Angular, Astro, Nuxt, and Vite framework handlers for consistency.
• Comprehensive Testing: New unit tests have been added for getBuildScript, and existing tests for warnIfCustomBuildScript have been updated to reflect the changes.

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[gemini-code-assist]

Code Review

This pull request adds support for the next build --webpack flag, which is a valuable addition. The refactoring to use a central getBuildScript utility is well-executed. However, I've identified a critical issue in the Next.js implementation that would prevent the feature from working as intended, along with some minor issues in the updated tests. My review includes suggestions to address these points.