There's a combination pack you can buy where the base game and expansion are bundled together, but otherwise you have to buy both separately (and that combination pack doesn't include the Collector's Edition).

The Au Ra race is the only major feature of the expansion that will be available to new players right away, though. All the other major features are locked behind finishing the current ARR game first. So if you'd rather play as one of the existing races rather than Au Ra, then you're probably better off just getting A Realm Reborn for now and waiting until you reach the end of it before deciding whether to get Heavensward. But who knows how many minor features and perks will be available with the expansion. We won't get all the details until it comes out, only the major features they're using to promote it.

(Personally, I'm rather hoping they include quite a few features that would make Heavensward beneficial even to new players. As it stands, most of their marketing is focused on stuff that's only meaningful to long-time players who would likely buy it anyway. I'm not sure if the new race is by itself enough of a draw to get people who can't use any of its other features to buy it.)