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

## 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 single-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/snancusumpw`

## 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 @stdlib/blas/ext/base packages and their native C implementations, then compare how cumulative-sum operations are organized. Confirm the intended behavior for single-precision strided arrays, NaN omission, and pairwise summation. Done means the new snancusumpw package is implemented and its behavior is verified.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.