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 #207 (24f1b73) into master (9fe2381) will not change coverage. The diff coverage is 80.00%.

@@           Coverage Diff           @@
##           master     #207   +/-   ##
=======================================
  Coverage   76.66%   76.66%           
=======================================
  Files          10       10           
  Lines         360      360           
  Branches      112      112           
=======================================
  Hits          276      276           
  Misses         73       73           
  Partials       11       11           

Impacted Files	Coverage Δ
lib/interpolateName.js	95.00% <66.66%> (ø)
lib/getHashDigest.js	89.39% <100.00%> (ø)

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