Write a function that groups a string into parentheses clusters. Each cluster should be balanced.
split("()()()") ➞ ["()", "()", "()"]
split("((()))") ➞ ["((()))"]
split("((()))(())()()(()())") ➞ ["((()))", "(())", "()", "()", "(()())"]
split("((())())(()(()()))") ➞ ["((())())", "(()(()()))"]
( must exist with its matching closing parens ) in the same cluster.