test(ember): Enable ESLint rule to enforce ES5 in vendor scripts

Author: rreckonerr

packages/ember/.eslintignore

@@ -1,6 +1,5 @@
 # unconventional js
 /blueprints/*/files/
-/vendor/
 
 # compiled output
 /dist/

packages/ember/.eslintrc.js

@@ -4,6 +4,20 @@ module.exports = {
   extends: ['../../.eslintrc.js'],
 
   overrides: [
+    {
+      // Vendor scripts are injected as inline <script> tags and must use ES5 syntax
+      // to ensure compatibility with older browsers that cannot transpile inline scripts.
+      // Setting ecmaVersion: 5 ensures ESLint will error on ANY ES6+ syntax at parse time.
+      files: ['vendor/**/*.js'],
+      parserOptions: {
+        sourceType: 'script',
+        ecmaVersion: 5,
+      },
+      env: {
+        browser: true,
+        node: false,
+      },
+    },
     {
       // addon files
       files: ['{addon,app,tests}/**/*.{js,ts,d.ts}'],