QuickSort : Instead of taking pivot as 1st element, we should take the pivot as last element, due to time complexity


Instead of this
`const pivotElement = array.shift();` // O(n)

we should use 
`const pivotElement = array.pop();` // O(1)

When taking the pivot as last element 2 test cases are failing.
1. QuickSort › should do stable sorting
2. QuickSort › should visit NOT SORTED array element specified number of times

