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/Bytecode/Op.cpp

@@ -678,7 +678,7 @@ ThrowCompletionOr<void> SuperCall::execute_impl(Bytecode::Interpreter& interpret
     // NOTE: This is implied by the strong C++ type.
 
     // 11. Perform ? InitializeInstanceElements(result, F).
-    TRY(vm.initialize_instance_elements(*result, f));
+    TRY(result->initialize_instance_elements(f));
 
     // 12. Return result.
     interpreter.accumulator() = result;