Add exercise index as property of the exercise

    #execute = null

    #context = new ExerciseExecutionContext()

    constructor(name, execute) {

    constructor(name, index, execute) {

        this.name = name

        this.index = index

        // Bind the execute function to the ExerciseExecutionContext.

        // This means that the `this` keyword, when used in the function, will

        // point to this ExerciseExecutionContext object.

         * Note: This must be a function declared with the function keyword, arrow functions (()=>{...}) do not work.

         */

        exercise(name, execute) {

            const exerciseObj = new Exercise(name, execute)

            const exerciseObj = new Exercise(name, this.exercises.length, execute)

            this.exercises.push(exerciseObj)

        }

    }