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.

首先在 Code 中重现所请求的 Ctrl+D 行为，并将其与 Ctrl+G 以及 issue 中描述的现有限制（多光标）进行比较。当 Ctrl+D 选择下一个匹配实例，同时保留之前的所有选择时，即表示完成；请在编辑器中确认该行为。

由索引模型根据 Issue 内容生成。