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

Merging #65 (2af4f1c) into master (861834c) will not change coverage. The diff coverage is n/a.

@@            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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