LibJS: Constructor function's "prototype" property should be writable

This matches other engines.

Libraries/LibJS/AST.cpp

@@ -759,7 +759,7 @@ Value ClassExpression::execute(Interpreter& interpreter, GlobalObject& global_ob
         prototype->define_property(vm.names.constructor, class_constructor, 0);
         if (interpreter.exception())
             return {};
-        class_constructor->define_property(vm.names.prototype, prototype, 0);
+        class_constructor->define_property(vm.names.prototype, prototype, Attribute::Writable);
         if (interpreter.exception())
             return {};
         class_constructor->set_prototype(super_constructor.is_null() ? global_object.function_prototype() : &super_constructor.as_object());

Libraries/LibJS/Runtime/ScriptFunction.cpp

@@ -71,7 +71,7 @@ void ScriptFunction::initialize(GlobalObject& global_object)
     if (!m_is_arrow_function) {
         Object* prototype = vm.heap().allocate<Object>(global_object, *global_object.new_script_function_prototype_object_shape());
         prototype->define_property(vm.names.constructor, this, Attribute::Writable | Attribute::Configurable);
-        define_property(vm.names.prototype, prototype, 0);
+        define_property(vm.names.prototype, prototype, Attribute::Writable);
     }
     define_native_property(vm.names.length, length_getter, nullptr, Attribute::Configurable);
     define_native_property(vm.names.name, name_getter, nullptr, Attribute::Configurable);

Libraries/LibJS/Tests/functions/function-prototype-writable.js

@@ -0,0 +1,10 @@
+test("a function's prototype property should be writable", () => {
+    function x() {}
+    var desc = Object.getOwnPropertyDescriptor(x, "prototype");
+    expect(desc.writable).toBe(true);
+    expect(desc.enumerable).toBe(false);
+    expect(desc.configurable).toBe(false);
+
+    x.prototype = 1;
+    expect(x.prototype).toBe(1);
+});