feat: refactor string truncate-middle to support grapheme clusters and code points

[ujjwalv01]

Resolves part of #1062 (truncate-middle portion).

Description

What is the purpose of this pull request?

This pull request:

• Adds @stdlib/string/base/truncate-middle-code-points which truncates the middle Unicode code points of a string.
• Adds @stdlib/string/base/truncate-middle-grapheme-clusters which truncates the middle grapheme clusters of a string.
• Refactors @stdlib/string/truncate-middle to act as a mode dispatcher, supporting grapheme (default), code_point, and code_unit modes via an options.mode parameter.

Related Issues

Does this pull request have any related issues?

This pull request has the following related issues:

• Part of Tracking issue: refactor string packages handling grapheme clusters in terms of "base" packages #1062

Questions

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No.

Other

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This PR supersedes the previously abandoned PR #1119. All changes requested by the reviewer in that PR (pluralizing the package name, making seq a mandatory argument in base packages) have been incorporated.

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[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
string/base/truncate-middle-code-points	$\\color{green}134/134$ $\\color{green}+100.00\\%$	$\\color{green}9/9$ $\\color{green}+100.00\\%$	$\\color{green}1/1$ $\\color{green}+100.00\\%$	$\\color{green}134/134$ $\\color{green}+100.00\\%$
string/base/truncate-middle-grapheme-clusters	$\\color{green}155/155$ $\\color{green}+100.00\\%$	$\\color{green}12/12$ $\\color{green}+100.00\\%$	$\\color{green}1/1$ $\\color{green}+100.00\\%$	$\\color{green}155/155$ $\\color{green}+100.00\\%$
string/truncate-middle	$\\color{red}178/182$ $\\color{green}+97.80\\%$	$\\color{red}17/19$ $\\color{green}+89.47\\%$	$\\color{green}1/1$ $\\color{green}+100.00\\%$	$\\color{red}178/182$ $\\color{green}+97.80\\%$

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

[kgryte]

@ujjwalv01 It is actually better if you split the packages into separate PRs. Why? Because we perform squash merging, and, for purposes of the changelog, we want one package added per commit.

Obviously, the top-level package PR will be dependent on the corresponding base packages, but that is fine. Just open the top-level package after the base packages have been merged.

[ujjwalv01]

@kgryte Understood! That makes complete sense for the changelog. I will focus on getting the two base package PRs merged first. We can leave this one on draft until those are merged, and then I'll rebase this branch so it only contains the top-level modifications.