RFC: add BLAS routine `isamax`

## 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 to add a routine to find the index of the first element having maximum absolute value in a single-precision floating-point vector, as defined in [BLAS Auxiliary][blas-aux]. Specifically adding `@stdlib/blas/base/isamax` is proposed.

## 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

[blas-aux]: http://www.netlib.org/lapack/explore-html/d0/d73/group__aux__blas.html

Start by reviewing the proposed @stdlib/blas/base/isamax package and the linked BLAS Auxiliary definition for the expected routine behavior. Determine the package structure and validation requirements from nearby BLAS routines, then confirm that the new routine finds the first maximum absolute value index for a single-precision vector and has the corresponding tests and documentation.

Written by the indexing model from the issue text.