DEV: Skip babel for qunit and sinon (#26028)

Skipping babel for qunit is part of the default embroider blueprint. Adding these doesn't seem to have a measurable effect on build time, but it does stop this message from being printed in the build log:

```
The code generator has deoptimised the styling of /Users/david/discourse/discourse/node_modules/sinon/pkg/sinon-esm.js as it exceeds the max of 500KB
```

app/assets/javascripts/discourse/ember-cli-build.js

@@ -228,6 +228,14 @@ module.exports = function (defaults) {
         ],
       },
     },
+    skipBabel: [
+      {
+        package: "qunit",
+      },
+      {
+        package: "sinon",
+      },
+    ],
   });
 
   return mergeTrees([appTree, mergeTrees(extraPublicTrees)]);