Fix babel preset require path for Shakapacker 9.3.0 with explicit .js extension

The shakapacker babel preset requires the explicit .js extension in the require
statement for Jest/Babel to resolve it correctly. Without the extension, Jest
fails with "Cannot find module" error.

Changes:
- Restored .js extension in spec/dummy/babel.config.js
- Added eslint-disable-next-line comment to suppress import/extensions rule

Error fixed:
  Cannot find module '/path/to/shakapacker/package/babel/preset'
  at babel.config.js:1:27

This is a special case where the .js extension is required for module resolution
in the Babel/Jest context, even though ESLint normally discourages it.

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

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

Author: justin808

spec/dummy/babel.config.js

@@ -1,4 +1,5 @@
-const defaultConfigFunc = require('shakapacker/package/babel/preset');
+// eslint-disable-next-line import/extensions
+const defaultConfigFunc = require('shakapacker/package/babel/preset.js');
 
 module.exports = function createBabelConfig(api) {
   const resultConfig = defaultConfigFunc(api);