[RFC]: skew-normal distribution

### Description

This RFC proposes the [skew-normal distribution](https://en.wikipedia.org/wiki/Skew_normal_distribution)

### Related Issues

I didn't see any.

### Questions

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### Other

No.

### Checklist

- [X] I have read and understood the [Code of Conduct](https://github.com/stdlib-js/stdlib/blob/develop/CODE_OF_CONDUCT.md).
- [X] Searched for existing issues and pull requests.
- [X] The issue name begins with `RFC:`.