feat(forms): add hasError and getError to AbstractControlDirective (#11985)

Allows cleaner expressions in template-driven forms.

Before:

    <label>Username</label><input name="username" ngModel required #username="ngModel">
    <div *ngIf="username.dirty && username.control.hasError('required')">Username is required</div>

After:

    <label>Username</label><input name="username" ngModel required #username="ngModel">
    <div *ngIf="username.dirty && username.hasError('required')">Username is required</div>

Fixes #7255

tools/public_api_guard/forms/index.d.ts

@@ -84,6 +84,8 @@ export declare abstract class AbstractControlDirective {
     valid: boolean;
     value: any;
     valueChanges: Observable<any>;
+    getError(errorCode: string, path?: string[]): any;
+    hasError(errorCode: string, path?: string[]): boolean;
     reset(value?: any): void;
 }