bug(web): KMWString substring methods do not properly match behavior of native String equivalents

> Interestingly, I found a few discrepancies on a few of the utility methods:
>
> .kmwSlice and the substring methods don't quite align with the full spec for their native String counterparts as specified on MDN. They are "fine" as far as our existing use-cases for the methods go, though.

Posted by @jahorton in the description of #13719.

Notes can be seen in comments in their corresponding unit tests introduced by that PR.

I _believe_ that we don't currently use the cases that aren't properly being handled, so this is likely low priority. We should be aware that this is a thing, though, in case we ever do start using one of the mismatching cases.