LibJS: Implement bytecode generation for For-In/Of statements

This also implements the rather interesting behaviour that #12772 relies
on, so this fixes that bug in BC mode (the AST interp remains affected).

Userland/Libraries/LibJS/Bytecode/Generator.cpp

@@ -194,7 +194,10 @@ CodeGenerationErrorOr<void> Generator::emit_store_to_reference(JS::ASTNode const
         return {};
     }
 
-    VERIFY_NOT_REACHED();
+    return CodeGenerationError {
+        &node,
+        "Unimplemented/invalid node used a reference"sv
+    };
 }
 
 String CodeGenerationError::to_string()