RFC: add the Skellam distribution

## Checklist

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- [x] Read and understood the [Code of Conduct][code-of-conduct].
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- [x] If this is a feature request, the issue name begins with `RFC:`.

## Description

> Description of the issue (or feature request).

This RFC proposes adding the [Skellam distribution](https://en.wikipedia.org/wiki/Skellam_distribution). Similar to other distribution namespaces, adding support for the Skellam distribution would entail a family of packages for evaluating the PMF, CDF, mean, median, etc.

Namespace: `@stdlib/stats/base/dists/skellam`
Reference: [`@stdlib/stats/base/dists/*`](https://github.com/stdlib-js/stdlib/tree/develop/lib/node_modules/%40stdlib/stats/base/dists)

## Related Issues

> Does this issue (or feature request) have any related issues?

No.

## Questions

> Any questions for reviewers?

No.

## Other

> Any other information relevant to this issue (or feature request)? This may include screenshots, references, stack traces, sample output, and/or implementation notes.

Reference implementations:

- [SciPy](https://docs.scipy.org/doc/scipy/reference/generated/scipy.stats.skellam.html#scipy.stats.skellam)

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

[faq]: https://github.com/stdlib-js/stdlib/blob/develop/FAQ.md