Multi-cursor mode

In some other editors (at least Atom) you can have multiple cursors (with `Ctrl`+click, or `Ctrl`+`Shift`+`↓`, and when you type something, text is inserted under both of them. It is really useful when you have to perform many times the same modification on various lines.


Issue 中没有确定任何文件、测试或入口点。首先定位编辑器的光标和文本输入处理，然后检查当前编辑器组件是否支持多个光标。完成的标准是支持所描述的多光标创建和同步文本插入。

由索引模型根据 Issue 内容生成。