feat: add ndarray/flatten-from-by

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Resolves stdlib-js/metr-issue-tracker#115.

Description

What is the purpose of this pull request?

This pull request:

• add ndarray/flatten-from-by

Related Issues

Does this pull request have any related issues?

This pull request has the following related issues:

• Resolves [RFC]: add ndarray/flatten-from-by metr-issue-tracker#115

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• Read, understood, and followed the contributing guidelines.

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Utilized AI for most of the things in the checklist above.

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[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
ndarray/flatten-from-by	$\color{red}360/367$ $\color{green}+98.09%$	$\color{red}32/36$ $\color{green}+88.89%$	$\color{green}1/1$ $\color{green}+100.00%$	$\color{red}360/367$ $\color{green}+98.09%$

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

[kgryte]

Suggested change

	> Flatten an [ndarray][@stdlib/ndarray/ctor] from a specified dimension while applying a callback function to each element.
	> Flatten an [ndarray][@stdlib/ndarray/ctor] according to a callback function starting from a specified dimension.

Applies here and throughout this PR.

[kgryte]

Overall, this is looking good. Initial comment only concerns updating the descriptions to better align with what we already use in flatten-from and flatten-by.