62. implement BigInt addition

easy  - accepted / - tried

Luckily we have BigInt in JavaScript so handle big numbers.

What if we need to do it by ourselves for older browsers?

You are asked to implement a string addition function, with all non-negative integers in string.

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

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

What is time & space complexity of your approach?