76. implement BigInt addition with sign

This is a follow-up on 62. implement BigInt addition.

You are asked to implement a string addition function, with possible negative integers. Also, '+' plus sign should also be supported

add('-999999999999999999', '-1')
// '-1000000000000000000'

add('-999999999999999999', '+1')
// '-999999999999999998'

Don't use BigInt directly, it is not our goal here.