refactor: update string/truncate to dispatch to base packages based on mode

[ujjwalv01]

Progresses #1062

Description

What is the purpose of this pull request?

This pull request:

• Refactors @stdlib/string/truncate to act as a mode-based dispatcher, aligning with the standard stdlib string API structure.
• Adds an options.mode parameter supporting grapheme (default), code_point, and code_unit string truncation types.
• Delegates execution to the respective base packages (base/truncate, base/truncate-code-point, and base/truncate-grapheme-cluster).

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.

Note: This is a Draft PR because it relies on the pending merge of three prerequisite base packages:

1. string/base/truncate-grapheme-cluster
2. string/base/truncate-code-point
3. string/base/truncate

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 scaffold the options.mode dispatching boilerplate and to expand the test/types coverage across the three new base modes.

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@stdlib-js/reviewers

[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
string/base/truncate-code-point	$\\color{red}139/142$ $\\color{green}+0.00\\%$	$\\color{red}12/13$ $\\color{green}+0.00\\%$	$\\color{green}1/1$ $\\color{green}+0.00\\%$	$\\color{red}139/142$ $\\color{green}+0.00\\%$
string/base/truncate-grapheme-cluster	$\\color{green}133/133$ $\\color{green}+0.00\\%$	$\\color{green}9/9$ $\\color{green}+0.00\\%$	$\\color{green}1/1$ $\\color{green}+0.00\\%$	$\\color{green}133/133$ $\\color{green}+0.00\\%$
string/base/truncate	$\\color{green}119/119$ $\\color{green}+0.00\\%$	$\\color{green}7/7$ $\\color{green}+0.00\\%$	$\\color{green}1/1$ $\\color{green}+0.00\\%$	$\\color{green}119/119$ $\\color{green}+0.00\\%$
string/truncate	$\\color{red}184/186$ $\\color{green}+0.00\\%$	$\\color{red}19/20$ $\\color{green}+0.00\\%$	$\\color{green}1/1$ $\\color{green}+0.00\\%$	$\\color{red}184/186$ $\\color{green}+0.00\\%$

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

[ujjwalv01]

This is a Draft PR for the top-level string/truncate refactor. Because this package acts as a dispatcher for the base utilities, I have temporarily merged the following three branches into this one for local testing and CI verification purposes:

• feat-string-base-truncate-grapheme-cluster
• feat-string-base-truncate-code-point
• feat-string-base-truncate

Once those three upstream PRs are reviewed and merged into develop, I will rebase this branch to clean up the commit history so that only the changes for the top-level @stdlib/string/truncate package remain.