Add support for class member variable

supersedes #1417
Another option would be to have:

```js
transformPropertyVariableDeclaration(tree) {
  return new AnonBlock(null, [getter, setter]);
}
```

and then these gets expanded for free in transformList.

The name AnonBlock has so far only been used for statements but I don't see a reason why it would not work for any tree. Maybe it should be renamed to something more suitable?