Fix ESLint import/extensions error in babel.config.js

Removed explicit .js extension from shakapacker preset import to comply
with ESLint's import/extensions rule.

Error was:
  1:35  error  Unexpected use of file extension "js" for
  "shakapacker/package/babel/preset.js"  import/extensions

Changed:
  require('shakapacker/package/babel/preset.js')
To:
  require('shakapacker/package/babel/preset')

🤖 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.js');
+const defaultConfigFunc = require('shakapacker/package/babel/preset');
 
 module.exports = function createBabelConfig(api) {
   const resultConfig = defaultConfigFunc(api);