feat(forms): allow markAsPending to emit events (#20212)

closes #17958

BREAKING CHANGE:
- `AbstractControl#statusChanges` now emits an event of `'PENDING'` when you call `AbstractControl#markAsPending`
- Previously it did not emit an event when you called `markAsPending`
- To migrate you would need to ensure that if you are filtering or checking events from `statusChanges` that you account for the new event when calling `markAsPending`

PR Close #20212

packages/forms/src/model.ts

@@ -357,10 +357,18 @@ export abstract class AbstractControl {
 
   /**
    * Marks the control as `pending`.
+   *
+   * An event will be emitted by `statusChanges` by default.
+   *
+   * Passing `false` for `emitEvent` will cause `statusChanges` to not event an event.
    */
-  markAsPending(opts: {onlySelf?: boolean} = {}): void {
+  markAsPending(opts: {onlySelf?: boolean, emitEvent?: boolean} = {}): void {
     (this as{status: string}).status = PENDING;
 
+    if (opts.emitEvent !== false) {
+      (this.statusChanges as EventEmitter<any>).emit(this.status);
+    }
+
     if (this._parent && !opts.onlySelf) {
       this._parent.markAsPending(opts);
     }