feat: add @stdlib/string/base/truncate-middle-grapheme-clusters

[ujjwalv01]

Resolves part of #1062

Description

What is the purpose of this pull request?

This pull request:

• Adds the @stdlib/string/base/truncate-middle-grapheme-clusters package.
• Implements a base utility function that truncates a string in the middle to a specified length based on grapheme clusters.
• Ensures that complex visual characters spanning multiple Unicode code points (such as emojis with modifiers) are not split when truncating strings.
• Serves as the default dispatching target for the top-level @stdlib/string/truncate-middle package.

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

Any questions for reviewers of this pull request?

No.

Other

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No.

Checklist

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• Read, understood, and followed the contributing guidelines.

AI Assistance

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• No

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• Code generation (e.g., when writing an implementation or fixing a bug)
• Test/benchmark generation
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I utilized an AI assistant to automate the generation of test cases and benchmark scaffolding. The core logic implementation and integration were authored manually.

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@stdlib-js/reviewers

[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
string/base/truncate-middle-grapheme-clusters	$\\color{green}155/155$ $\\color{green}+100.00\\%$	$\\color{green}12/12$ $\\color{green}+100.00\\%$	$\\color{green}1/1$ $\\color{green}+100.00\\%$	$\\color{green}155/155$ $\\color{green}+100.00\\%$

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

[ujjwalv01]

Closing this PR in favor of #13782 , which consolidates all three packages (truncate-middle-code-points, truncate-middle-grapheme-clusters, and the top-level truncate-middle refactor) into a single PR for easier review. All changes from this PR are included there.

[kgryte]

Reopening, as our preference is that PRs adding new packages only contain a single package.

[ujjwalv01]

Thanks for reopening! These are up-to-date and ready for review.