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.

Comienza reproduciendo el comportamiento solicitado de Ctrl+D en Code y compáralo con Ctrl+G y con la limitación existente de múltiples cursores descrita en el issue. Se considera terminado cuando Ctrl+D selecciona la siguiente instancia coincidente mientras conserva todas las selecciones anteriores; confirma el comportamiento en el editor.

Escrito por el modelo de indexación a partir del texto del issue.