Milestone3 : Add & remove

[abdessalem1998]

• Remove all hardcoded items from the tasks array.
• Create a new JavaScript file for the new functionality.
• Implement a function for adding a new task (add a new element to the array).
• Implement a function for editing task descriptions.
• Implement a function for deleting a task (remove an element from the array).
• Implement a function for the "Clear all completed" button (use filter() method).
• By default new tasks should have the property completed set to false and the property index set to the value of the new array length (i.e. if you're adding a 5th task to the list, the index of that task should equal to 5).
• Deleting a task should update all remaining items' indexes, so they represent the current list order and are unique.
• All changes to the To Do List should be saved in local storage.