events: correctly trigger the onDidAddListener event in emitter options (#230259)

* Correctly trigger the `onDidAddListener` event in emitter options

* Add test for `Emitter.onDidAddListener`

src/vs/base/common/event.ts

@@ -1155,6 +1155,7 @@ export class Emitter<T> {
 			} else {
 				this._listeners.push(contained);
 			}
+			this._options?.onDidAddListener?.(this);
 
 			this._size++;
 

src/vs/base/test/common/event.test.ts

@@ -308,6 +308,27 @@ suite('Event', function () {
 		assert.strictEqual(lastCount, 1);
 	});
 
+	test('onDidAddListener', () => {
+		let count = 0;
+		const a = ds.add(new Emitter({
+			onDidAddListener() { count += 1; }
+		}));
+
+		assert.strictEqual(count, 0);
+
+		let subscription = ds.add(a.event(function () { }));
+		assert.strictEqual(count, 1);
+
+		subscription.dispose();
+		assert.strictEqual(count, 1);
+
+		subscription = ds.add(a.event(function () { }));
+		assert.strictEqual(count, 2);
+
+		subscription.dispose();
+		assert.strictEqual(count, 2);
+	});
+
 	test('onWillRemoveListener', () => {
 		let count = 0;
 		const a = ds.add(new Emitter({