textarea 根据光标位置添加内容

 // 获取焦点

      let txt = document.getElementById("countRule");

      let temp = txt.value;

      txt.focus();

      let pointIndex = txt.selectionStart;

      let str1 = temp.substr(0,pointIndex);

      let str2 = temp.substr(pointIndex,temp.length);

      txt.value = str1+dom+str2;

      if( site != 'no' ){

        let newIndex = pointIndex + site;

        txt.setSelectionRange(newIndex,newIndex);

      }