RFC: add BLAS(-like) routine `daxpby`

## 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 to add a [BLAS-like][blas-level-1] routine to compute `a*x + b*y` as defined in [MKL][mkl]. Specifically adding `@stdlib/blas/base/daxpby` is proposed.

## Related Issues

> Does this feature request have any related issues?

No.

## Questions

> Any questions for reviewers?

No.

## Other

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

No.

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

[blas-level-1]: http://www.netlib.org/blas/#_level_1

[mkl]: https://software.intel.com/en-us/mkl-developer-reference-c-cblas-axpby#005DD24D-A872-4A42-909C-5E7458191670