fix: export all validator directives

packages/core/src/public_api.ts

@@ -6,6 +6,7 @@ export * from './module';
 export * from './validator-loader.service';
 export * from './validator.class';
 export * from './directives/form-validator.directive';
+export * from './directives/validators';
 export * from './directives/form-submit.directive';
 export * from './strategies';
 export { NgxValidators } from './validators';