chore: replace deprecated String.prototype.substr()

[CommanderRoot]

This PR contains a:

• bugfix
• new feature
• 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

[alexlamsl]

This doesn't seem to be fixing any bugs − unless you can demonstrate with a test case that fails before and passes after this PR.

Thanks for taking the time, although as you stated these whimsical deprecations never results in any removal (see __proto__) so I don't see the need, unless this gives measureable performance boost on some platforms (without regression in others).

[CommanderRoot]

It removes a deprecation warning in the IDE. But you're right, unless the function gets removed by browsers or other software which uses this code there is nothing broken.
As for performance, I have not tested it directly but from looking at other repos it's basically the same.

[lbesecker195]

Checked each of the seven replacements: substr(pos, len) becomes slice(pos, pos + len), substr(1, n - 1) becomes slice(1, n), and single-argument calls are unchanged in meaning. All equivalent for the non-negative indices used here. LGTM.