LibJS: Make ObjectPrototype an immutable prototype exotic object

To make this happen, this patch implements the SetImmutablePrototype
abstract operation (as a method on Object) and then overrides
[[SetPrototypeOf]] on ObjectPrototype.

Userland/Libraries/LibJS/Tests/builtins/Object/Object.prototype.js

@@ -3,3 +3,10 @@ test("basic functionality", () => {
     Object.prototype.foo = 123;
     expect(o.foo).toBe(123);
 });
+
+test("is an immutable prototype exotic object", () => {
+    const p = Object.create(null);
+    expect(() => {
+        Object.setPrototypeOf(Object.prototype, p);
+    }).toThrowWithMessage(TypeError, "Object's [[SetPrototypeOf]] method returned false");
+});