Remove assignability cases in getNarrowedType + an isArray improvement for readonly arrays (#39258)

* Explore using a different isArray declaration

* Add tests and the new isArray definition

* Baseline updates

* Upda the isArray type

src/lib/es5.d.ts

@@ -1376,7 +1376,7 @@ interface ArrayConstructor {
     (arrayLength?: number): any[];
     <T>(arrayLength: number): T[];
     <T>(...items: T[]): T[];
-    isArray(arg: any): arg is any[];
+    isArray<T>(arg: T | {}): arg is T extends readonly any[] ? (unknown extends T ? never : readonly any[]) : any[];
     readonly prototype: any[];
 }