add string to represent path

```js
{
 foo: {
   bar: {
      baz: {
          [generic.PathString]: true
      }
    }
  }
}
```

this will add a string, so that the output looks like:

```js
{
 foo: {
   bar: {
    baz: {
       "@path": "foo.bar.baz"
     }
    }
  }
}
```

then users can pass this path in their HTTP requests and can type-check and at runtime as well.
