feat(language-service): provide completion for $event variable (#34570)

This commit adds a completion for the `$event` variable in bound event
expressions.

This is the first in a series of PRs to support completions for
properties on `$event` variables (https://github.com/angular/vscode-ng-language-service/issues/531).

PR Close #34570

packages/language-service/src/completions.ts

@@ -316,6 +316,7 @@ function attributeValueCompletions(info: AstResult, htmlPath: HtmlAstPath): ng.C
     templatePath.tail.visit(visitor, null);
     return visitor.results;
   }
+
   // In order to provide accurate attribute value completion, we need to know
   // what the LHS is, and construct the proper AST if it is missing.
   const htmlAttr = htmlPath.tail as Attribute;

packages/language-service/test/completions_spec.ts

@@ -753,6 +753,16 @@ describe('completions', () => {
       expectContain(completions, CompletionKind.VARIABLE, ['$event']);
     });
   });
+
+  describe('$event completions', () => {
+    it('should suggest $event in event bindings', () => {
+      mockHost.override(TEST_TEMPLATE, `<div (click)="myClick(~{cursor});"></div>`);
+      const marker = mockHost.getLocationMarkerFor(TEST_TEMPLATE, 'cursor');
+      debugger;
+      const completions = ngLS.getCompletionsAt(TEST_TEMPLATE, marker.start);
+      expectContain(completions, CompletionKind.VARIABLE, ['$event']);
+    });
+  });
 });
 
 function expectContain(