Fix babel preset import path for Shakapacker 9.3.0

Add .js extension to shakapacker/package/babel/preset import path.
This is required for Shakapacker 9.3.0's package.json exports field
to correctly resolve the module path.

Fixes Jest test failures:
- tests/react-on-rails.import.test.js
- tests/react-on-rails.require.test.js

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

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

Author: justin808

spec/dummy/babel.config.js

@@ -1,4 +1,4 @@
-const defaultConfigFunc = require('shakapacker/package/babel/preset');
+const defaultConfigFunc = require('shakapacker/package/babel/preset.js');
 
 module.exports = function createBabelConfig(api) {
   const resultConfig = defaultConfigFunc(api);