The iterated square root of a number is the number of times the square root function must be applied to bring the number strictly under 2.
Given an integer, return its iterated square root. Return "invalid" if it is negative.
i_sqrt(1) ➞ 0
i_sqrt(2) ➞ 1
i_sqrt(7) ➞ 2
i_sqrt(27) ➞ 3
i_sqrt(256) ➞ 4
i_sqrt(-1) ➞ "invalid"
Idea for iterated square root by Richard Spence.