refactor: replace deprecated String.prototype.substr()

[CommanderRoot]

String.prototype.substr() is deprecated so we replace it with String.prototype.slice() which works similarily but isn't deprecated.
.substr() probably isn't going away anytime soon but the change is trivial so it doesn't hurt to do it.

PR Checklist

• npm run lint passes
• Inline documentation is included / updated
• Unit tests are included / updated

[welcome]

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[montoyamoraga]

@all-contributors please add @CommanderRoot for code

[allcontributors]

@montoyamoraga

I've put up a pull request to add @CommanderRoot! 🎉

[limzykenneth]

Looks good. Thanks!