fix: replace deprecated String.prototype.substr()

[CommanderRoot]

Summary

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.

Test plan (required)

I tested the return of each statement to make sure it's the same as before

[Sharcoux]

Can be tested in @cantoo/html2canvas