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.

Checklist

• PR contains only changes related; no stray files, etc.
• README updated (where applicable)
• Tests written (where applicable)

[codecov]

Codecov Report

Merging #1942 (3cf0f84) into master (c1b21a9) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #1942   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          102       102           
  Lines         2085      2085           
  Branches       472       472           
=========================================
  Hits          2085      2085           

Impacted Files	Coverage Δ
src/lib/isDataURI.js	100.00% <100.00%> (ø)
src/lib/isEmail.js	100.00% <100.00%> (ø)
src/lib/isIdentityCard.js	100.00% <100.00%> (ø)
src/lib/isTaxID.js	100.00% <100.00%> (ø)
src/lib/isURL.js	100.00% <100.00%> (ø)

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[rubiin]

Looks good