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;
 }
 

Userland/Libraries/LibJS/Interpreter.cpp

@@ -63,6 +63,8 @@ void Interpreter::run(GlobalObject& global_object, const Program& program)
     // At this point we may have already run any queued promise jobs via on_call_stack_emptied,
     // in which case this is a no-op.
     vm.run_queued_promise_jobs();
+
+    vm.finish_execution_generation();
 }
 
 GlobalObject& Interpreter::global_object()

Userland/Libraries/LibJS/Runtime/VM.h

@@ -166,6 +166,9 @@ public:
     bool underscore_is_last_value() const { return m_underscore_is_last_value; }
     void set_underscore_is_last_value(bool b) { m_underscore_is_last_value = b; }
 
+    u32 execution_generation() const { return m_execution_generation; }
+    void finish_execution_generation() { ++m_execution_generation; }
+
     void unwind(ScopeType type, FlyString label = {})
     {
         m_unwind_until = type;
@@ -279,6 +282,8 @@ private:
     Shape* m_scope_object_shape { nullptr };
 
     bool m_underscore_is_last_value { false };
+
+    u32 m_execution_generation { 0 };
 };
 
 template<>

Userland/Libraries/LibWeb/DOM/Document.cpp

@@ -655,6 +655,8 @@ JS::Interpreter& Document::interpreter()
                     dbgln("  {} at {}:{}:{}", function_name, source_range.filename, source_range.start.line, source_range.start.column);
                 }
             }
+
+            vm.finish_execution_generation();
         };
         m_interpreter = JS::Interpreter::create<Bindings::WindowObject>(vm, *m_window);
     }