RFC: add `stats/strided/nancumin`

## 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 minimum value of a strided array ignoring `NaN` values. This would be a "generic" lower-level strided interface with the following package name:

Package: `@stdlib/stats/strided/nancumin`

## 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

Start by reviewing the proposed package name, @stdlib/stats/strided/nancumin, and nearby strided statistics packages in the repository. Define the package's cumulative-minimum behavior for strided arrays while ignoring NaN values, then add the corresponding implementation and tests once the RFC's design is resolved.

Written by the indexing model from the issue text.