Time limit does not work if running prepends.

[blackening]

https://github.com/source-academy/js-slang/blob/64c636f8ce283970ad7aa0c5b4cf6606053073c3/src/index.ts#L459

Based on this, the time-limit will only adjust upwards if the two programs are the same.

If there's a prepend, the time limit will not be raised.

Suggest moving time limit control to cadet-frontend, instead of relying on stateful re-running on js-slang.

Note that this essentially prevents the raising of time limit for paths with prepend.