LibJS: More properly implement scoping rules in bytecode codegen

Now we emit CreateVariable and SetVariable with the appropriate
initialization/environment modes, much closer to the spec.
This makes a whole lot of things like let/const variables, function
and variable hoisting and some other things work :^)

Userland/Libraries/LibJS/Runtime/GeneratorObject.h

@@ -29,7 +29,7 @@ private:
     ExecutionContext m_execution_context;
     ECMAScriptFunctionObject* m_generating_function { nullptr };
     Value m_previous_value;
-    Bytecode::RegisterWindow m_frame;
+    Optional<Bytecode::RegisterWindow> m_frame;
     bool m_done { false };
 };