Implements the `browse/new-releases` endpoint

[hayribakici]

This PR adds the browse/new-releases endpoint.

Since browsing categories in categories.dart and browsing new releases share the same v1/browse/ api-path, I let the class Categories inherit from the Browse class:

What do you guys think of this approach? After looking into the code, I figured, that the Playlist class shares the same v1/browse _path like in the Browse class, although it is only used twice in Playlist (one in featured and one byCategoryId()). I wonder, if we should clean this up and figure a proper model of the library. Maybe an Issue regarding this topic should be discussed. Or we get rid of the Browse class completely and put the new-realeases somewhere else.