--- Splits a string into a table of substrings, using a separator string.
--- Stops splitting after n substrings have been found or the end of the string is reached.
---
--- @param str string The string to split.
--- @param sep string The separator string to use for splitting.
--- @param n number The maximum number of substrings to return.
--- @return A table of strings containing the substrings.
function string_splitn(str: string, sep: string, n: number) -> {string} end