feat: add string/base/truncate-grapheme-cluster

[ujjwalv01]

Progresses #1062

Description

What is the purpose of this pull request?

This pull request:

• Adds @stdlib/string/base/truncate-grapheme-cluster, a new base package that truncates strings to a specified length in grapheme clusters.
• Provides a foundational utility necessary for refactoring the top-level @stdlib/string/truncate package to support grapheme clusters.

Related Issues

Does this pull request have any related issues?

This pull request has the following related issues:

• 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

Any other information relevant to this pull request? This may include screenshots, references, and/or implementation notes.

This is the first of several PRs to address the string/truncate refactor. The implementation utilizes @stdlib/string/num-grapheme-clusters and @stdlib/string/next-grapheme-cluster-break to chunk the string accurately by user-perceived visual characters without native input validation, matching standard base package paradigms.

Checklist

Please ensure the following tasks are completed before submitting this pull request.

• Read, understood, and followed the contributing guidelines.

AI Assistance

When authoring the changes proposed in this PR, did you use any kind of AI assistance?

• Yes
• No

If you answered "yes" above, how did you use AI assistance?

• Code generation (e.g., when writing an implementation or fixing a bug)
• Test/benchmark generation
• Documentation (including examples)
• Research and understanding

Disclosure

If you answered "yes" to using AI assistance, please provide a short disclosure indicating how you used AI assistance. This helps reviewers determine how much scrutiny to apply when reviewing your contribution. Example disclosures: "This PR was written primarily by Claude Code." or "I consulted ChatGPT to understand the codebase, but the proposed changes were fully authored manually by myself.".

I utilized an AI coding assistant to help scaffold the test cases and benchmark boilerplate for this package. The implementation and tests were subsequently manually verified and adjusted by me locally.

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Coverage Report

Package	Statements	Branches	Functions	Lines
string/base/truncate-grapheme-cluster	$\\color{green}133/133$ $\\color{green}+0.00\\%$	$\\color{green}9/9$ $\\color{green}+0.00\\%$	$\\color{green}1/1$ $\\color{green}+0.00\\%$	$\\color{green}133/133$ $\\color{green}+0.00\\%$

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