chore: replace deprecated String.prototype.substr()

[CommanderRoot]

This PR contains a:

• [x] bugfix
• [ ] new feature
• [x] code refactor
• [ ] test update
• [ ] typo fix
• [ ] metadata update

Motivation / Use-Case

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.

Breaking Changes

None

Additional Info

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (861834c) to head (2af4f1c). Report is 47 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #65 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 2 2 Lines 12 12 Branches 3 3 ========================================= Hits 12 12 ```

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