feat: add ndarray/flatten

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Resolves stdlib-js/metr-issue-tracker#75.

Description

What is the purpose of this pull request?

This pull request:

• add ndarray/flatten

Related Issues

Does this pull request have any related issues?

This pull request:

• resolves [RFC]: add ndarray/flatten metr-issue-tracker#75
• progresses [RFC]: achieve ndarray API parity with built-in JavaScript arrays  #2656

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• Read, understood, and followed the contributing guidelines.

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Coverage Report

Package	Statements	Branches	Functions	Lines
ndarray/flatten	$\color{green}408/408$ $\color{green}+100.00\%$	$\color{green}26/26$ $\color{green}+100.00\%$	$\color{green}1/1$ $\color{green}+100.00\%$	$\color{green}408/408$ $\color{green}+100.00\%$

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

[kgryte]

Also, similar to NumPy (ref: https://numpy.org/doc/stable/reference/generated/numpy.ravel.html#numpy-ravel), we should support specifying a "read" order.

flatten( x[, opts] )

with options

• depth: default infinity.
• order: read order. If row-major, always flatten an input array in lexicographic order. If column-major, always flatten in colexicographic order. If same, flatten in the order of the input array (i.e., equivalent to order = getOrder( x )). If any, flatten in colexicographic order if input array data is column-major contiguous; otherwise, flatten in lexicographic order (i.e., flatten according to how the data is arranged in memory, which may not correspond to the stated array order).

[kgryte]

Additional references:

• Order options: https://github.com/stdlib-js/stdlib/tree/develop/lib/node_modules/%40stdlib/ndarray/array
• "colexicographic" terminology: https://github.com/stdlib-js/stdlib/tree/develop/lib/node_modules/%40stdlib/array/base/flatten

[kgryte]

Left an initial review. This implementation needs some significant surgery before it can move forward.

[kgryte]

LGTM after clean-up.