JS/Frontend app.store multi response sets should be collections

## Feature Request

It would be great if we can make result sets collections. So that we not just get an array of objects, but also their respective meta data and possible helper functions.

**Is your feature request related to a problem? Please describe.**

The problem right now is that if you return meta information like pagination counts etc, this information is dropped while pulling objects from the store.

**Describe the solution you'd like**
It would be cool pushing the data into a collection that also supports meta information.

**Justify why this feature belongs in Flarum's core, rather than in a third-party extension**

Only core has a store and handles the api requests using `app.request`.