Consolidate Rspack config into webpack directory with conditionals

Restructured to follow the pattern from react_on_rails_demo:
- Moved all Rspack-specific changes into config/webpack/ files
- Added bundler auto-detection using config.assets_bundler
- Uses conditional logic to support both Webpack and Rspack
- Removed config/rspack/ directory

Benefits:
- Easier to compare Webpack vs Rspack configurations
- All changes in one place with clear conditionals
- Smaller diff - shows exactly what's different for Rspack
- Follows react_on_rails best practices

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: justin808