LibJS: Make JS parser emit accurate this insights for constructors

This way we don't have to handle it when instantiating the constructor.

Libraries/LibJS/Parser.h

@@ -39,6 +39,7 @@ struct FunctionNodeParseOptions {
         IsGeneratorFunction = 1 << 6,
         IsAsyncFunction = 1 << 7,
         HasDefaultExportName = 1 << 8,
+        IsConstructor = 1 << 9,
     };
 };