Daily Challenge #119 - Adding Big Numbers

https://dev.to/thepracticaldev/daily-challenge-119-adding-big-numbers-2bjo

We need to sum big numbers and we require your help.

Write a function that returns the sum of two numbers. The input numbers are strings and the function must return a string.

Example:

add("0", "0"); -> "0"
add("123", "321"); -> "444"
add("11", "99"); -> "110"
add('12345678910123456789', '987654323109876543210');
  -> "1000000002019999999999"

Notes:

• The input numbers are big.
• The input is a string of only digits
• The numbers are positives