Support to stage 3

[pabloalmunia]

It's important the next TC39 meeting session about Decorator Metadata for Stage 3 (slides, spec). We need the decorator metadata because, without it, we have limited the decorator feature.

Especially important is - at least for some teams - to be able to coordinate decorators with metadata, especially the class member decorators, which in this current pattern as mere metadata annotators, and the associated class decorator, which reads from the metadata and performs the necessary transformations and adaptations.

As a workaround, when we use "method", "getter", "setter" or "accessor" decorators, we can put a symbol in each function to put metadata there, but with "field" we don't have a shelf where to put metadata.

Of course, without this feature, work with metadata and inheritance is very complicated.

Consequently, we need the decorator metadata as soon as possible.