[Enhancement] Add namespace definition for helper

#### What are you trying to achieve?
Can we try to add namespace definition for helper. Because when we are writing helpers, there might be the same action for different scenarios. For example, I want to. add something to my favorite in desktop version or mobile version. But I don't want to write 2 different methods with different name like `addFavoriteInDesktop` and `addFavoriteInMobile`. I hope that I can have `addFavorite` methods in both `favorite_helper.js` in `desktop` folder and `favorite_helper.js` in `mobile` folder. But they have different namespace like
```
class FavoriteHelper extends Helper {
_namespace() {
return 'desktop';
}

addFavorite(something) {...}
}
```

Then I can use it in the test case like
```
Namespace('desktop');
Feature('Favorite');
Scenario('Add something to my favorite', (I) => {
I.addFavorite(something);
.....
}
```

Review the proposed favorite_helper.js files in the desktop and mobile folders, along with the Helper and Namespace entry points referenced by the issue. Determine how _namespace() and Namespace('desktop') should interact with feature scenarios, then define tests showing that identical helper methods can be selected by namespace. Done means both desktop and mobile helpers remain independently callable as addFavorite.

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