find either the next upper or next lower case character index in string str, based on the numeric option,
starting from index start and optionally ending at exclusive-index end.
if your selected uppercase/lowercase option is not found, or if your option === 0, then undefined is returned.
find either the next upper or next lower case character index in string
str, based on the numericoption, starting from indexstartand optionally ending at exclusive-indexend. if your selected uppercase/lowercase option is not found, or if youroption === 0, thenundefinedis returned.this function is just a composition of the functions findNextUpperCase and findNextLowerCase.