Index Multiplier
Return
the sum of all items in a list, where each item is multiplied by its index
(zero-based). For empty lists, return 0.
Examples
index_multiplier([1, 2, 3, 4, 5]) ➞ 40
# (1*0 + 2*1 + 3*2 + 4*3 + 5*4)
index_multiplier([-3, 0, 8, -6]) ➞ -2
# (-3*0 + 0*1 + 8*2 + -6*3)
