[Proposal] add "split" function - `array[string] split(string $separator, string $subject)`

[morokin]

Just copy of issue opened by @biochimia: https://github.com/jmespath/jmespath.site/issues/35

I propose the addition of a split() function, counterpart to the already standard join() function. split breaks a string into an array of substrings that are separated by $separator in the original $subject.

(Not entirely unlike Python's string.split() function)