refactor: update `blas/ext/base/snansum` to follow current project convention

[XueSongTap]

…ention

Resolves #1528 .

Description

What is the purpose of this pull request?

This pull request:

• [x] Refactors @stdlib/blas/ext/base/snansum to follow current project conventions.
• [x] Migrates from C++ add-on interfaces to C add-on interfaces.
• [x] Makes various style and simplification changes.

Related Issues

Does this pull request have any related issues?

This pull request:

• resolves #1528.

Questions

Any questions for reviewers of this pull request?

No.

Other

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

No.

Checklist

Please ensure the following tasks are completed before submitting this pull request.

• [x] Read, understood, and followed the contributing guidelines.

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