Fix shadow fn to return a function for on modifier

The shadowed fn must return a function since its return value is
passed to the on modifier as the callback argument.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: NullVoxPopuli

packages/@glimmer-workspace/integration-tests/test/keywords/fn-test.ts

@@ -61,6 +61,7 @@ class KeywordFn extends RenderTest {
   'can be shadowed'(assert: Assert) {
     let fn = () => {
       assert.step('shadowed:success');
+      return () => {};
     };
 
     let greet = () => {};