Write a function that flattens an Array of Array objects into a flat Array. Your function must only do one level of flattening.
flatten([1,2,3]) // => [1,2,3]
flatten([[1,2,3],["a","b","c"],[1,2,3]]) // => [1,2,3,"a","b","c",1,2,3]
flatten([[[1,2,3]]]) // => [[1,2,3]]flatten [1,2,3] # => [1,2,3]
flatten [[1,2,3],["a","b","c"],[1,2,3]] # => [1,2,3,"a","b","c",1,2,3]
flatten [[[1,2,3]]] # => [[1,2,3]]flatten [1,2,3] # => [1,2,3]
flatten [[1,2,3],["a","b","c"],[1,2,3]] # => [1,2,3,"a","b","c",1,2,3]
flatten [[[1,2,3]]] # => [[1,2,3]]flatten [1,2,3] # => [1,2,3]
flatten [[1,2,3],["a","b","c"],[1,2,3]] # => [1,2,3,"a","b","c",1,2,3]
flatten [[[1,2,3]]] # => [[1,2,3]]