Write a function that when given a number >= 0, returns an Array of ascending length subarrays.
pyramid(0) => [ ]
pyramid(1) => [ [1] ]
pyramid(2) => [ [1], [1, 1] ]
pyramid(3) => [ [1], [1, 1], [1, 1, 1] ]
Note: the subarrays should be filled with 1s