RFC: add support for generating pseudorandom numbers using XORSHIFT-ADD

## Checklist

> Please ensure the following tasks are completed before submitting a feature request.

- [x] Read and understood the [Code of Conduct][code-of-conduct].
- [x] Searched for existing issues and pull requests.
- [x] The issue name begins with `RFC:`.

## Description

> Description of the feature request.

This RFC proposes adding supporting for generating pseudorandom numbers using the [XORSHIFT-ADD algorithm](https://github.com/MersenneTwister-Lab/XSadd).

Package: `@stdlib/random/base/xorshift-add`
Alias: `xorshiftadd`

## Related Issues

> Does this feature request have any related issues?

No.

## Questions

> Any questions for reviewers?

Some background research may be necessary to determine whether XORSHIFT-ADD has any problems, such as correlated random variates or other design flaws.

## Other

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

- [C implementation](https://github.com/MersenneTwister-Lab/XSadd)
- [Homepage](http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/XSADD/)
- [Comparison to other XORSHIFT variants](http://vigna.di.unimi.it/ftp/papers/xorshiftplus.pdf)

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