Add --rspack option to react_on_rails:install generator

[justin808]

Summary

Add a --rspack option to the react_on_rails:install generator to create demos configured for Rspack instead of Webpack by default.

Motivation

Rspack provides 10-20x faster build times compared to Webpack while maintaining API compatibility. The demos/basic-v16 demo (PR #20) demonstrates a complete working implementation with:

• Rspack configuration that works seamlessly with React on Rails
• Unified webpack/rspack configuration approach
• Proper HMR and React Fast Refresh setup
• Complete migration documentation

This PR proves that React on Rails works excellently with Rspack and provides a template for the generator.

Proposed Implementation

Add a --rspack flag to the install generator:

rails generate react_on_rails:install --rspack

Changes needed:

1. Package dependencies: Install rspack packages instead of webpack

   • @rspack/core, @rspack/cli, @rspack/plugin-react-refresh
   • rspack-manifest-plugin
   • Use SWC instead of Babel by default

2. shakapacker.yml configuration:

   default:
     assets_bundler: 'rspack'
     javascript_transpiler: 'swc'

3. Webpack configuration: Use conditional bundler detection (already demonstrated in basic-v16)

Reference Implementation

PR #20 (demos/basic-v16) provides a complete reference implementation including:

• Correct package.json dependencies for rspack
• Unified config/webpack/ configuration that works for both bundlers
• Comprehensive migration documentation
• bin/switch-bundler script for easy bundler switching

Benefits

• 10-20x faster builds for new React on Rails projects
• Better developer experience with faster HMR
• Future-proof - Rspack is gaining widespread adoption
• Easy migration path - Users can still switch back to webpack if needed

Additional Context

The basic-v16 demo includes:

• Full migration guide at docs/webpack-to-rspack-migration.md
• Performance benchmarks (250ms vs 3.5s builds)
• Troubleshooting documentation
• Script for switching between bundlers

This generator option would help new projects benefit from Rspack's performance immediately while maintaining the flexibility to use Webpack if needed.