checks whether the specified task exists
| id | string | number | the task id | 
| boolean | true, if such a task exists. Otherwise, false | 
gantt.addTask({
    id:10,
    text:"Task #5",
    start_date:"02-09-2023",
    duration:28
}, "project_2");
 
gantt.isTaskExists(10); // ->true
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