feat(compiler-cli): add a locale option to ng-xi18n

Fixes #12303
Closes #14537

modules/@angular/compiler/test/i18n/serializers/xtb_spec.ts

@@ -186,8 +186,8 @@ export function main(): void {
       });
 
       it('should throw when trying to save an xtb file',
-         () => { expect(() => { serializer.write([]); }).toThrowError(/Unsupported/); });
+         () => { expect(() => { serializer.write([], null); }).toThrowError(/Unsupported/); });
 
     });
   });
-}
+}