feat(core): add NO_ERRORS_SCHEMA that allows any properties to be set on any element (#10956)

Often it is useful to test a component without rendering certain directives/components
in its template because these directives require some complicated setup.

You can do that by using NO_ERRORS_SCHEMA.

TestBed.configureTestingModule({
  schemas: [NO_ERRORS_SCHEMA]
});

This would disable all schema checks in your tests.

tools/public_api_guard/core/index.d.ts

@@ -840,6 +840,9 @@ export declare class NgZone {
     static isInAngularZone(): boolean;
 }
 
+/** @experimental */
+export declare const NO_ERRORS_SCHEMA: SchemaMetadata;
+
 /** @stable */
 export declare class NoAnnotationError extends BaseException {
     constructor(typeOrFunc: Type<any> | Function, params: any[][]);