Given a list of numbers, representing the height of a mountain in certain intervals, return whether this mountain is scalable.
A mountain can be considered scalable if each number is within 5 units of the next number in either direction.
is_scalable([1, 2, 4, 6, 7, 8]) ➞ True
is_scalable([40, 45, 50, 45, 47, 52]) ➞ True
is_scalable([2, 9, 11, 10, 18, 21]) ➞ False
The list may start at any number and can be any length.