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

tools/public_api_guard/forms/forms.d.ts

@@ -38,6 +38,7 @@ export declare abstract class AbstractControl {
     }): void;
     markAsPending(opts?: {
         onlySelf?: boolean;
+        emitEvent?: boolean;
     }): void;
     markAsPristine(opts?: {
         onlySelf?: boolean;