feat: add blas/ext/index-of

[headlessNode]

Progresses #2656.

Description

What is the purpose of this pull request?

This pull request:

• add blas/ext/index-of

Related Issues

Does this pull request have any related issues?

This pull request:

• progresses [RFC]: achieve ndarray API parity with built-in JavaScript arrays  #2656

Questions

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No.

Other

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Checklist

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• Read, understood, and followed the contributing guidelines.

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[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
blas/ext/index-of	$\color{red}656/666$ $\color{green}+98.50\%$	$\color{red}74/78$ $\color{green}+94.87\%$	$\color{green}3/3$ $\color{green}+100.00\%$	$\color{red}656/666$ $\color{green}+98.50\%$

The above coverage report was generated for the changes in this PR.

[kgryte]

I've updated the implementation. The only thing remaining are updating the tests. I suggest you take this on, as this will be a good exercise in both verifying my changes and understanding the changes made.

[kgryte]

LGTM after some additional clean-up.

@headlessNode Re: tests. Notice how I updated the fromIndex test cases. Using a fromIndex of 0 is not informative, as that is the default value, so you were not actually testing whether a from index was supported.

Further notice how I added comments illustrating array shapes which makes interpreting expected values easier to infer.