nesting score

Write a function, nesting_score, that takes in a string of brackets as an argument. The function should return the score of the string according to the following rules:

- [] is worth 1 point
- XY is worth m + n points where X, Y are substrings of well-formed brackets and m, n are their respective scores
- [S] is worth 2 * k points where S is a substring of well-formed brackets and k is the score of that substring

You may assume that the input only contains well-formed square brackets.

test_00

nesting_score("[]") # -> 1

test_01

nesting_score("[][][]") # -> 3

test_02

nesting_score("[[]]") # -> 2

test_03

nesting_score("[[][]]") # -> 4

test_04

nesting_score("[[][][]]") # -> 6

test_05

nesting_score("[[][]][]") # -> 5

test_06

nesting_score("[][[][]][[]]") # -> 7

test_07

nesting_score("[[[[[[[][]]]]]]][]") # -> 129

test_08

nesting_score("") # -> 0