feat: add truncate-middle string package

[steff456]

Part of https://github.com/stdlib-js/stdlib/issues/1062

Description

What is the purpose of this pull request?

This pull request:

• Adds @stdlib/string/truncate-middle

Related Issues

Does this pull request have any related issues?

This pull request:

• Part of https://github.com/stdlib-js/stdlib/issues/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 PR depends on,

• https://github.com/stdlib-js/stdlib/pull/1118
• https://github.com/stdlib-js/stdlib/pull/1119
• https://github.com/stdlib-js/stdlib/pull/1120

Checklist

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

• [x] Read, understood, and followed the contributing guidelines.

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