Create a function which calculates how many numbers are missing from an ordered number line. This number line starts at the first value of the array, and increases by 1 to the end of the number line, ending at the last value of the array.
howManyMissing([1, 2, 3, 8, 9]) ➞ 4
// The number line starts at 1 and ends at 9 (so the numbers 0 and 10 aren't missing from it).
// The numbers missing from this line are 4, 5, 6, and 7.
// 4 numbers are missing.
howManyMissing([1, 3]) ➞ 1
howManyMissing([7, 10, 11, 12]) ➞ 2
howManyMissing([1, 3, 5, 7, 9, 11]) ➞ 5
howManyMissing([5, 6, 7, 8]) ➞ 0
If the number line is complete, or the array is empty, return 0.