Migrate shelljs to execa (#12)

[mattfsourcecode]

Migrate from shelljs to execa to Resolve Dependency Warnings

Why this change?

Installing the module previously displayed the following warnings:

npm WARN deprecated inflight@1.0.6: This module is not supported, and leaks memory. Do not use it. Check out lru-cache if you want a good and tested way to coalesce async requests by a key value, which is much more comprehensive and powerful.
npm WARN deprecated glob@7.2.3: Glob versions prior to v9 are no longer supported

These warnings were caused by shelljs, which depends on outdated packages (inflight and glob@7). While shelljs maintainers have stated this does not pose a security risk, removing these deprecated dependencies is preferable for long-term maintainability.

What was changed?

• Replaced shelljs with execa, a modern and well-maintained alternative for running shell commands in Node.js.
• Updated all references to shell.exec(), shell.cd(), and shell.rm() with equivalent execa or fs functions.
• Updated devDependencies.

Benefits of execa over shelljs

✅ Actively maintained and widely used.
✅ Better error handling and debugging capabilities.
✅ No reliance on deprecated dependencies.

This migration removes unnecessary package warnings and ensures the project remains up-to-date with maintained dependencies.

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Closes #12