Create a function that returns the sum of all even elements in a 2D matrix.
sumOfEvens([
[1, 0, 2],
[5, 5, 7],
[9, 4, 3]
]) ➞ 6
// 2 + 4 = 6
sumOfEvens([
[1, 1],
[1, 1]
]) ➞ 0
sumOfEvens([
[42, 9],
[16, 8]
]) ➞ 66
sumOfEvens([
[],
[],
[]
]) ➞ 0