Improve SWC compiler detection and configuration for Shakapacker 9.3.0+

[justin808]

Summary

Improve the SWC compiler detection and default behavior for projects using Shakapacker 9.3.0+.

Background

PR #1896 (now closed) contained valuable SWC improvements that should be extracted and applied to current master.

Proposed Changes

1. Enhance using_swc? Helper

• Update to properly parse shakapacker.yml for SWC configuration
• Make SWC the default for Shakapacker 9.3.0+
• Better fallback logic when config is ambiguous

2. Update Generator

• Set SWC as default in generated shakapacker.yml for Shakapacker 9.3.0+
• Add helpful comments in generated config
• Ensure swc-loader and @swc/core are in devDependencies

3. Documentation

• Document SWC as the recommended compiler for Shakapacker 9.3.0+
• Add migration notes for projects upgrading from Babel

Files to Update

From PR #1896 as reference:

• lib/generators/react_on_rails/base_generator.rb
• lib/generators/react_on_rails/generator_helper.rb
• lib/generators/react_on_rails/templates/base/base/config/shakapacker.yml
• Package.json files (move SWC deps to devDependencies)

Testing

• Test generator with Shakapacker 9.3.0
• Test SWC detection logic
• Verify build works with SWC
• Test fallback to Babel when SWC not configured

Related

• Extracted from closed PR Upgrade Shakapacker to 9.3.0 #1896
• Related to Shakapacker 9.3.0 upgrade (already on master)

Labels

enhancement