LibJS: Move initialize_instance_elements() from VM to Object

This makes more sense as an Object method rather than living within the
VM class for no good reason. Most of the other 7.3.xx AOs already work
the same way.
Also add spec comments while we're here.

Userland/Libraries/LibJS/Runtime/Object.cpp

@@ -606,6 +606,27 @@ ThrowCompletionOr<void> Object::define_field(ClassFieldDefinition const& field)
     return {};
 }
 
+// 7.3.33 InitializeInstanceElements ( O, constructor ), https://tc39.es/ecma262/#sec-initializeinstanceelements
+ThrowCompletionOr<void> Object::initialize_instance_elements(ECMAScriptFunctionObject& constructor)
+{
+    // 1. Let methods be the value of constructor.[[PrivateMethods]].
+    // 2. For each PrivateElement method of methods, do
+    for (auto const& method : constructor.private_methods()) {
+        // a. Perform ? PrivateMethodOrAccessorAdd(O, method).
+        TRY(private_method_or_accessor_add(method));
+    }
+
+    // 3. Let fields be the value of constructor.[[Fields]].
+    // 4. For each element fieldRecord of fields, do
+    for (auto const& field : constructor.fields()) {
+        // a. Perform ? DefineField(O, fieldRecord).
+        TRY(define_field(field));
+    }
+
+    // 5. Return unused.
+    return {};
+}
+
 // 10.1 Ordinary Object Internal Methods and Internal Slots, https://tc39.es/ecma262/#sec-ordinary-object-internal-methods-and-internal-slots
 
 // 10.1.1 [[GetPrototypeOf]] ( ), https://tc39.es/ecma262/#sec-ordinary-object-internal-methods-and-internal-slots-getprototypeof