Configure Babel transpiler for Shakapacker 9.1.0 in Pro dummies

Shakapacker 9.1.0 defaults to SWC transpiler, but the Pro dummy apps
don't have swc-loader installed. Configure them to use Babel instead
by adding javascript_transpiler: babel to shakapacker.yml.

This fixes the build error:
"Your Shakapacker config specified using swc, but swc-loader package
is not installed."

Updated:
- react_on_rails_pro/spec/dummy/config/shakapacker.yml
- react_on_rails_pro/spec/execjs-compatible-dummy/config/shakapacker.yml

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

Co-Authored-By: Claude <[email protected]>

Author: justin808

react_on_rails_pro/spec/dummy/config/shakapacker.yml

@@ -6,6 +6,7 @@ default: &default
   public_root_path: public
   public_output_path: packs
   nested_entries: true
+  javascript_transpiler: babel
 
   cache_path: tmp/cache/webpacker
   check_yarn_integrity: false

react_on_rails_pro/spec/execjs-compatible-dummy/config/shakapacker.yml

@@ -7,6 +7,7 @@ default: &default
   public_output_path: packs
   cache_path: tmp/webpacker
   webpack_compile_output: true
+  javascript_transpiler: babel
 
   # Additional paths webpack should lookup modules
   # ['app/assets', 'engine/foo/app/assets']