RFC: add `blas/ext/base/dnancusumkbn2`

## 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 support for computing the cumulative sum of a double-precision floating-point strided array ignoring `NaN` values and using a second-order iterative Kahan-Babuska algorithm. This would be a lower-level strided interface which includes a native C implementation and has the following package name:

Package: `@stdlib/blas/ext/base/dnancusumkbn2`

## Related Issues

> Does this feature request have any related issues?

None.

## 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

Start by locating the existing BLAS extension packages and the entry point for `@stdlib/blas/ext/base/dnancusumkbn2`; no source files or tests are named in the RFC. Done means the package provides a native C implementation of a strided double-precision cumulative sum that ignores NaN values and uses the specified second-order Kahan-Babuska algorithm.

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