Update Fragment blueprint to use class instead of extend

As Ember team is deprecating EmberObjects and it's recommending to use ES6 classes, I think we can update the Fragment blueprint to follow this syntax:

```
import MF from 'ember-data-model-fragments';

export default class <% name %> extends MF.Fragment {
<% attrs %>
}
```

What do you think? I may leave a PR with my proposal.

Locate the Fragment blueprint and inspect its current generated template, then compare it with the class-based example in the issue. Done means the blueprint generates Fragment files using the shown class and extends syntax; no test or file path is named in the issue.

Escrita pelo modelo de indexação a partir do texto da issue.