RFC: add BLAS routine `idamax`

## 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 double-precision floating-point vector, as defined in [BLAS Auxiliary][blas-aux]. Specifically adding `@stdlib/blas/base/idamax` 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