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

## 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 pairwise summation. 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/dnancusumpw`

## 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 with the RFC description and the proposed package entry point, @stdlib/blas/ext/base/dnancusumpw. Determine how the lower-level strided interface, native C implementation, NaN handling, and pairwise summation fit together; done means the package supports cumulative sums for double-precision strided arrays while ignoring NaN values.

Scritto dal modello di indicizzazione a partire dal testo della issue.