fix: `String.splitOn` bug (#3832)

Fixes #3829. As reported on Zulip (both
[recently](https://leanprover.zulipchat.com/#narrow/stream/270676-lean4/topic/current.20definition.20of.20.60String.2EsplitOn.60.20is.20incorrect/near/430930535)
and [a year
ago](https://leanprover.zulipchat.com/#narrow/stream/270676-lean4/topic/should.20we.20redefine.20.60String.2EsplitOnAux.60.3F/near/365899332)),
`String.splitOn` has a bug when dealing with separators of more than one
character (which are luckily rare). The code change here is very small,
replacing a `i` with `i - j`, but it makes termination more complex so
that's where the rest of the line count goes.