[Feature]希望可以增加接口：通过X,Y轴的定位获取日期和任务行

What problem does this feature solve?

我希望vtable-gantt可以通过X,Y轴的定位获取日期和任务行，这样就可以支持自定义地拖动创建任务排期，另外点击创建排期目前还不能满足业务需求，使操作更加界面化，让用户使用方便

What does the proposed API look like?

gantt.dateFromPos(x, y);这样可以通过XY定位去获取相关信息的api，任务也效果相同，这样可以支持更高层面的定制化

    ///获取日期
    var ganttContainer = gantt.$task_data;
    var x = e.clientX - ganttContainer.getBoundingClientRect().left + ganttContainer.scrollLeft;
    var y = e.clientY - ganttContainer.getBoundingClientRect().top + ganttContainer.scrollTop;
    // 获取任务拖拽到的当前时间位置
    var currentDate = gantt.dateFromPos(x, y);
    if (!currentDate) {
      currentDate = new Date();
    }获取任务也大概如上写法

Start by locating the gantt implementation behind gantt.$task_data and the existing date-position handling used by the drag event example. Trace how X/Y coordinates map to timeline dates and task rows, then define and verify the proposed dateFromPos-style API for both results. Done means consumers can retrieve the date and task row from canvas coordinates for custom scheduling interactions.