Support "Select next" like Ctrl+D in Atom

This is probably related to multi-cursor mode, which is also not (yet) possible in the current version of Code. But a feature I really miss is the **Select next** or `ctrl+d`, just as it is implemented in most code editors. I's not the same as `ctrl+g`, where just the next instance is selected. The Select next feature actually selects the next instance while keeping the all previously selected instances also selected as well.