feat: add support for none to indicate no regularization

[nakul-krishnakumar]

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Resolves #None.

Description

What is the purpose of this pull request?

This pull request:

• Adds none value for penalties enum.

Related Issues

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This pull request has the following related issues:

• None.

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No.

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@stdlib-js/reviewers

[nakul-krishnakumar]

This PR currently blocks #13669 and #13670

[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
ml/base/sgd-classification/penalties	$\\color{green}148/148$ $\\color{green}+0.00\\%$	$\\color{green}5/5$ $\\color{green}+0.00\\%$	$\\color{green}2/2$ $\\color{green}+0.00\\%$	$\\color{green}148/148$ $\\color{green}+0.00\\%$
ml/base/sgd-classification/penalty-enum2str	$\\color{green}104/104$ $\\color{green}+0.00\\%$	$\\color{green}5/5$ $\\color{green}+0.00\\%$	$\\color{green}1/1$ $\\color{green}+0.00\\%$	$\\color{green}104/104$ $\\color{green}+0.00\\%$
ml/base/sgd-classification/penalty-str2enum	$\\color{green}96/96$ $\\color{green}+0.00\\%$	$\\color{green}5/5$ $\\color{green}+0.00\\%$	$\\color{green}1/1$ $\\color{green}+0.00\\%$	$\\color{green}96/96$ $\\color{green}+0.00\\%$

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

[kgryte]

This naming convention is a bit risky insofar as this doesn't clash with any other enum belonging to other parameterizations (e.g., loss functions, learning rates, etc).

Fine to leave as is for now. Otherwise, we may need to update our conventions if this assumption is invalidated in the future.

[kgryte]

LGTM after clean-up.