Supplementary plane code points are not handled atomically

cf. https://github.com/jmespath-community/jmespath.test/issues/2

String functions such as [`length`](https://jmespath.site/#func-length) and [`reverse`](https://jmespath.site/#func-reverse) and [`sort`](https://jmespath.site/#func-sort) are documented to treat strings as sequences of code points, but this implementation incorrectly interprets the code units of each surrogate pair representing a supplementary plane character as independent code points.