Convert a large int (e. g. 1234567890) into a string containing the usual decimal prefixes k, M, G, T, etc. for kilo, mega, giga, tera, etc. The result shall be groups of three digits, interspersed by the prefixes (e. g.
1G234M567k890), so that it is easy to glance the order of magnitude of the number without counting the digits.
It shall work for numbers of all digit counts up to 15. Only non-negative numbers are allowed as input.
More test cases:
0 → 0 123 → 123 1234 → 1k234 1000 → 1k000 1000000000 → 1G000M000k000