chore: 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.

Checklist

• npm run test passes
• .d.ts file is updated
• changelog is added, npm run add-change
• run bundle exec rake docs:api and inspect doc/latest/index.html if documentation is changed
• run npm run integration if integration test is changed
• non-code related change (markdown/git settings etc)

[github-actions]

Greetings! We’re closing this issue because it has been open a long time and hasn’t been updated in a while and may not be getting the attention it deserves. We encourage you to check if this is still an issue in the latest release and if you find that this is still a problem, please feel free to comment or open a new issue.

[CommanderRoot]

The PR is still relevant