LibJS: Store and maintain an "execution generation" counter

This counter is increased each time a synchronous execution sequence
completes, and will allow us to emulate the abstract operations
AddToKeptObjects & ClearKeptObjects efficiently.

Userland/Libraries/LibJS/Bytecode/Interpreter.cpp

@@ -141,6 +141,8 @@ Value Interpreter::run(Executable const& executable, BasicBlock const* entry_poi
     if (vm().call_stack().size() == 1)
         vm().pop_call_frame();
 
+    vm().finish_execution_generation();
+
     return return_value;
 }