RFC: Mann Whitney U test (Wilcoxon rank sum test)

## Checklist

- [x] Read and understood the [Code of Conduct][code-of-conduct].
- [x] Searched for existing issues and pull requests.
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## Description

> Perhaps it's possible to include the Mann-Whitney U test. I'm seeing the Kruskal test which is close!

For reference, in R this is:

```r
wilcox.test(var~grp, paired = FALSE)
```

[code-of-conduct]: https://github.com/stdlib-js/stdlib/blob/develop/CODE_OF_CONDUCT.md