JSProxyObject >> asJQuery, allows `domObject asJQuery`

Adds `asJQuery` to `JSObjectProxy`. This allows to send `asJQuery` to DOM element. It is useful for example in:

```smalltalk
jQuerySet each: [ :index :elem |
  elem asJQuery foo: bar
]
```

This was not possible (one must have used `(jQuery value: elem)`).