I didn’t realize this was a feature either.
@audiorush and others in this thread: If you look under Hotkeys in the preferences, you will find two commands. Swap line up and Swap line down. You can assign a shortcut key to those commands, or use the command palette to access them.
Like bernardo_v said, it might only be in the insider build. I’m using 0.8.4, so I can’t check the older versions.